feat(mobile): snap-scroll kanban board with viewport-sized columns

The pipelines board was already horizontally scrollable, but on a
phone it felt imprecise — swipes landed mid-column, there was no
visual hint that more columns existed past the viewport, and 260px
columns felt cramped at 375px width.

- Each StageColumn is now w-[85vw] min/max-clamped on mobile, so the
  next column's edge peeks in as a "more here" affordance. On lg+
  the columns revert to flex-1 basis-[260px] (share the row).
- Scroll container gets snap-x + snap-mandatory on mobile, snap-none
  on lg+. Each column has snap-start so swipes always land cleanly
  at the left edge of the next column.
- shrink-0 on mobile prevents flex from compressing the columns when
  space is tight.

No desktop visual change — all the mobile-specific classes are
overridden back by their lg: counterparts, and the original
flex-1 basis-[260px] behavior is preserved above lg.

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

src/components/pipelines/pipeline-board.tsx

@@ -102,7 +102,11 @@ export function PipelineBoard({
       onDragEnd={handleDragEnd}
       onDragCancel={handleDragCancel}
     >
-      <div className="pipeline-scroll flex gap-3 overflow-x-auto pb-4">
+      {/* snap-x + snap-mandatory on mobile so swipes land the next
+          stage cleanly at the viewport edge instead of mid-column.
+          Disabled on lg+ because the full board fits without scroll
+          there and snapping would interfere with the natural layout. */}
+      <div className="pipeline-scroll flex snap-x snap-mandatory gap-3 overflow-x-auto pb-4 lg:snap-none">
         {sortedStages.map((stage) => {
           const stageDeals = dealsByStage.get(stage.id) ?? [];
           const totalValue = stageDeals.reduce(
@@ -176,7 +180,13 @@ function StageColumn({
   const { setNodeRef, isOver } = useDroppable({ id: stage.id });
 
   return (
-    <div className="flex min-w-[260px] flex-1 basis-[260px] flex-col rounded-xl border border-slate-800 bg-slate-900/60 p-4">
+    // On mobile each column is `w-[85vw]` (with a reasonable min/max)
+    // so the next column's edge peeks in — a "there's more here" hint.
+    // snap-start lands each column cleanly when swiping. On lg+ we
+    // restore the flex-1 share-the-row behavior. The droppable ref is
+    // on the inner messages region below — intentionally NOT here, so
+    // a drag over the column header doesn't highlight the whole column.
+    <div className="flex w-[85vw] min-w-[260px] max-w-[320px] shrink-0 snap-start flex-col rounded-xl border border-slate-800 bg-slate-900/60 p-4 lg:w-auto lg:max-w-none lg:flex-1 lg:basis-[260px] lg:shrink lg:snap-none">
       {/* 3px colored top border — sits above the column's padding */}
       <div
         className="-mx-4 -mt-4 h-[3px] rounded-t-xl"