fix(theme): suppress hydration warning on root html data-theme (#207)

The `theme-boot` beforeInteractive script in layout.tsx rewrites
`data-theme` on <html> from localStorage before React hydrates, so for
any non-default theme the client DOM intentionally differs from the
server-rendered DEFAULT_THEME ("violet"). React then logged a hydration
mismatch on <html> on every load where the saved theme differed.

Add suppressHydrationWarning to the root <html>. It's scoped to that
element's own attributes (child mismatches still surface) and preserves
the documented localStorage-only/device-scoped theme design — unlike a
cookie-based SSR approach, which would change that behavior.

Verified in-browser (Next 16.2.6 / Turbopack): with a non-default theme
in localStorage, the reproduced "+ data-theme=violet / - data-theme=cobalt"
hydration error is gone; removing the prop brings it back (negative
control). typecheck, lint (0 errors), 370 tests pass.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

src/app/layout.tsx

@@ -70,6 +70,13 @@ export default function RootLayout({
       lang="en"
       data-theme={DEFAULT_THEME}
       className={`${inter.variable} h-full antialiased`}
+      // The `theme-boot` script below rewrites `data-theme` on <html>
+      // from localStorage before React hydrates, so for any non-default
+      // theme the client DOM intentionally differs from the server-
+      // rendered `DEFAULT_THEME`. suppressHydrationWarning silences the
+      // expected mismatch — it only applies to this element's own
+      // attributes, so genuine mismatches in children still surface.
+      suppressHydrationWarning
     >
       <head>
         <Script