feat(mobile): inbox single-pane flow on phone, unchanged on desktop

The inbox was a 3-pane flex row (list + thread + contact sidebar).
On a 375px phone both list and thread tried to share ~187px each
and the whole UI was unusable. Now:

- On <lg the inbox shows ONE pane at a time: list when nothing's
  selected, thread when a conversation is open.
- Thread gains a back-arrow in its header (mobile only) that pops
  back to the list and clears the ?c= URL param so refreshing lands
  on the list, not on the thread the user just backed out of.
- Conversation list grows to w-full on mobile (was fixed w-80)
  so each row uses the whole viewport width.
- Thread header tightens: name/phone truncate with min-w-0, session
  timer badge is hidden under sm to save room for back arrow + name.
- Dashboard shell's p-6 is already p-4 on mobile (PR #48); matched
  the -m-6 offset in the inbox to -m-4 sm:-m-6.

Desktop layout (lg+) is unchanged: list + thread + contact sidebar
side-by-side exactly as before.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/app/(dashboard)/inbox/page.tsx

@@ -10,6 +10,7 @@ import { MessageThread } from "@/components/inbox/message-thread";
 import { ContactSidebar } from "@/components/inbox/contact-sidebar";
 import { toast } from "sonner";
 import { WifiOff } from "lucide-react";
+import { cn } from "@/lib/utils";
 
 export default function InboxPage() {
   const router = useRouter();
@@ -191,6 +192,19 @@ export default function InboxPage() {
     [activeConversation?.id, router]
   );
 
+  // Mobile "back" — deselect the conversation so the list pane comes
+  // back. Also clears the ?c= param so a refresh lands on the list
+  // instead of re-opening the thread the user just backed out of.
+  const handleCloseConversation = useCallback(() => {
+    setActiveConversation(null);
+    setActiveContact(null);
+    setMessages([]);
+    // Clearing the ref lets the deep-link auto-selector fire again if
+    // the user later visits /inbox?c=<same-id> — desirable UX.
+    autoSelectedForDeepLinkRef.current = null;
+    router.replace("/inbox", { scroll: false });
+  }, [router]);
+
 
   const handleMessagesLoaded = useCallback((loaded: Message[]) => {
     setMessages(loaded);
@@ -224,8 +238,15 @@ export default function InboxPage() {
     [activeConversation]
   );
 
+  // On mobile (<lg) we show a SINGLE pane — either the list or the
+  // thread — rather than cramming both side-by-side. Selecting a
+  // conversation slides the thread in; the thread's back button pops
+  // it back to the list. On lg+ both panes render side-by-side as
+  // before, unchanged.
+  const hasActiveConv = !!activeConversation;
+
   return (
-    <div className="-m-6 flex h-[calc(100vh-3.5rem)] flex-col overflow-hidden">
+    <div className="-m-4 flex h-[calc(100vh-3.5rem)] flex-col overflow-hidden sm:-m-6">
       {/* WhatsApp connection banner — in the flex column, not absolute,
           so it pushes the panels down instead of overlapping them. */}
       {whatsappConnected === false && (
@@ -238,15 +259,33 @@ export default function InboxPage() {
       )}
 
       <div className="flex flex-1 overflow-hidden">
-        {/* Left panel: Conversation list */}
+        {/* Left panel: Conversation list.
+            Hidden on mobile when a conversation is selected so the
+            thread can occupy the full width. Always visible on lg+. */}
+        <div
+          className={cn(
+            "flex h-full flex-1 lg:flex-none",
+            hasActiveConv ? "hidden lg:flex" : "flex",
+          )}
+        >
           <ConversationList
             activeConversationId={activeConversation?.id ?? null}
             onSelect={handleSelectConversation}
             conversations={conversations}
             onConversationsLoaded={handleConversationsLoaded}
           />
+        </div>
 
-        {/* Center panel: Message thread */}
+        {/* Center panel: Message thread.
+            Hidden on mobile when no conversation is selected so the
+            list can occupy the full width. Always visible on lg+
+            (shows its own empty-state if no thread is picked yet). */}
+        <div
+          className={cn(
+            "flex h-full flex-1 lg:flex",
+            hasActiveConv ? "flex" : "hidden lg:flex",
+          )}
+        >
           <MessageThread
             conversation={activeConversation}
             contact={activeContact}
@@ -255,9 +294,11 @@ export default function InboxPage() {
             onNewMessage={handleNewMessage}
             onUpdateMessage={handleUpdateMessage}
             onStatusChange={handleStatusChange}
+            onBack={handleCloseConversation}
           />
+        </div>
 
-        {/* Right panel: Contact sidebar (hidden on small screens) */}
+        {/* Right panel: Contact sidebar — desktop only. */}
         <div className="hidden lg:block">
           <ContactSidebar contact={activeContact} />
         </div>

src/components/inbox/conversation-list.tsx

@@ -110,7 +110,10 @@ export function ConversationList({
   const activeFilter = FILTER_OPTIONS.find((o) => o.value === filter);
 
   return (
-    <div className="flex h-full w-80 flex-col border-r border-slate-800 bg-slate-900">
+    // w-full on mobile so the list occupies the whole viewport when it's
+    // the single pane showing; fixed 320px on desktop where it shares the
+    // row with the thread + contact sidebar.
+    <div className="flex h-full w-full flex-col border-r border-slate-800 bg-slate-900 lg:w-80">
       {/* Search + Filter */}
       <div className="space-y-2 border-b border-slate-800 p-3">
         <div className="relative">

src/components/inbox/message-thread.tsx

@@ -9,6 +9,7 @@ import {
   ChevronDown,
   UserPlus,
   Clock,
+  ArrowLeft,
 } from "lucide-react";
 import { format, isToday, isYesterday, differenceInHours } from "date-fns";
 import { Badge } from "@/components/ui/badge";
@@ -32,6 +33,13 @@ interface MessageThreadProps {
   onNewMessage: (message: Message) => void;
   onUpdateMessage: (id: string, updates: Partial<Message>) => void;
   onStatusChange: (conversationId: string, status: ConversationStatus) => void;
+  /**
+   * On mobile, the thread is shown full-screen with the conversation list
+   * hidden. This callback lets the page deselect the active conversation
+   * and reveal the list again. Rendered as a back-arrow in the header on
+   * mobile only.
+   */
+  onBack?: () => void;
 }
 
 function formatDateSeparator(dateStr: string): string {
@@ -72,6 +80,7 @@ export function MessageThread({
   onNewMessage,
   onUpdateMessage,
   onStatusChange,
+  onBack,
 }: MessageThreadProps) {
   const [loading, setLoading] = useState(false);
   const scrollRef = useRef<HTMLDivElement>(null);
@@ -278,20 +287,33 @@ export function MessageThread({
   return (
     <div className="flex flex-1 flex-col bg-slate-950">
       {/* Header */}
-      <div className="flex items-center justify-between border-b border-slate-800 bg-slate-900 px-4 py-3">
-        <div className="flex items-center gap-3">
-          <div className="flex h-9 w-9 items-center justify-center rounded-full bg-slate-700 text-sm font-medium text-white">
+      <div className="flex items-center justify-between gap-2 border-b border-slate-800 bg-slate-900 px-3 py-3 sm:px-4">
+        <div className="flex min-w-0 items-center gap-2 sm:gap-3">
+          {/* Back-to-list button — mobile only. Hidden on lg+ where the
+              conversation list is always visible next to the thread. */}
+          {onBack && (
+            <button
+              type="button"
+              onClick={onBack}
+              aria-label="Back to conversations"
+              className="flex h-9 w-9 flex-shrink-0 items-center justify-center rounded-md text-slate-300 hover:bg-slate-800 hover:text-white lg:hidden"
+            >
+              <ArrowLeft className="h-5 w-5" />
+            </button>
+          )}
+          <div className="flex h-9 w-9 flex-shrink-0 items-center justify-center rounded-full bg-slate-700 text-sm font-medium text-white">
             {displayName.charAt(0).toUpperCase()}
           </div>
-          <div>
-            <h2 className="text-sm font-semibold text-white">{displayName}</h2>
-            <p className="text-xs text-slate-400">{contact.phone}</p>
+          <div className="min-w-0">
+            <h2 className="truncate text-sm font-semibold text-white">{displayName}</h2>
+            <p className="truncate text-xs text-slate-400">{contact.phone}</p>
           </div>
-          {/* Session timer badge */}
+          {/* Session timer badge — hidden on the narrowest phones so
+              the name + back arrow keep their room. */}
           <Badge
             variant="outline"
             className={cn(
-              "ml-2 gap-1 border-slate-700 text-[10px]",
+              "ml-1 hidden gap-1 border-slate-700 text-[10px] sm:inline-flex sm:ml-2",
               sessionInfo.expired ? "text-red-400" : "text-emerald-400"
             )}
           >