feat: ship as a real iOS-installable PWA, restructure bottom nav, fix safe-area

- Add manifest.webmanifest with standalone display + warm-craft theme colors,
  apple-touch-icon, and 192/512/512-maskable icons (frame-with-sunset glyph).
- Add PWA meta tags + viewport-fit=cover so add-to-home-screen produces a
  true standalone app on iOS instead of a Safari bookmark.
- Drop the Shared bottom-nav tab; the in-page sub-tabs already cover that.
  Three nav tabs total (Home / Library / Settings); pending-share badge
  moves to the Library tab. Predicate-based isActive() now correctly
  disambiguates /library vs /library?tab=shared.
- Safe-area handling: bottom nav, bottom sheet, upload overlay, and #app
  respect env(safe-area-inset-*); sticky Library tabs anchor below the
  iPhone status bar. Introduces --bottom-nav-height token consumed by
  Settings, Library, and the toast.
- LibraryView reactively follows route.query.tab so deep-linking
  /library?tab=shared lands on the right sub-tab.
- Theme-color meta syncs client-side via useTheme.applyTheme so the
  user's chosen theme follows them into Android Chrome's chrome bar.

Test suite expanded to 278 tests / 100% line coverage (99.84% statements,
99.78% branches). Remaining gaps are unreachable defensive code.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/styles/global.scss

@@ -33,6 +33,9 @@
 
   // Touch target minimum
   --touch-min: 44px;
+
+  // Bottom nav clearance — 64px row + iOS home-indicator inset (zero on devices without one)
+  --bottom-nav-height: calc(64px + env(safe-area-inset-bottom));
 }
 
 // ─── Themes ──────────────────────────────────────────────────────────────────
@@ -158,6 +161,7 @@ body {
   min-height: 100dvh;
   display: flex;
   flex-direction: column;
+  padding-top: env(safe-area-inset-top);
 }
 
 // ─── Focus visible ────────────────────────────────────────────────────────────