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/composables/useTheme.ts

@@ -25,6 +25,12 @@ export function useTheme() {
   function applyTheme(themeId: string) {
     document.documentElement.dataset.theme = themeId
     if (auth.user) auth.user.theme = themeId
+
+    const theme = THEMES.find(t => t.id === themeId)
+    if (theme) {
+      const meta = document.querySelector('meta[name="theme-color"]')
+      if (meta) meta.setAttribute('content', theme.bg)
+    }
   }
 
   async function saveTheme(themeId: string) {