feat(flutter): present the library as Shadcn format tabs

[darwin67]

Summary

Layer 6 of the Shadcn stack (base: the reader toasts). Replaces the library's format filter buttons with a real ShadTabs page: the format tabs own the collection content.

• All/PDF/EPUB/CBZ are now ShadTabs; selecting one dispatches LibraryFormatChanged exactly as before.
• Tabs use maintainState: false so only the active collection is built (a non-selected ShadTab content is otherwise laid out without Expanded, which leaves a GridView unbounded), and scrollable: true so the bar doesn't overflow at 2× text scale and narrow widths.
• Library goldens regenerated for the tab bar.

No controller or model changes; this is presentation only.

Testing

• make test-flutter (284), make lint-flutter, make check-fmt-flutter.
• The three large-text/compact layout tests (reader settings remain scrollable, compact Japanese library at 200%, long multilingual metadata) pass via the scrollable tab bar.

Stack

1. brand theme + reader theme consolidation (feat(flutter): apply the Shosai brand accent and unify reader theming on Shadcn #109)
2. notice/Sonner infrastructure (feat(flutter): present controller notices as Sonner toasts #110)
3. import results → toasts (feat(flutter): report library import results as toasts #111)
4. reader transient failures → toasts (feat(flutter): present reader transient failures as toasts #112)
5. this PR — library format tabs
6. ShadContextMenu/ShadBadge/ShadSeparator
7. reader/dialog goldens + boundary docs

[darwin67]

Closing with owner approval as superseded by the Flutter UI restoration plan in docs/flutter-ui-restoration-plan.md (reference tooling landed in #117). This is not a claim that the Flutter replacement is already implemented. The branch is being preserved so useful work remains available.

The library-tab visual direction is superseded by Stage 3: wide sidebar and compact filter navigation, retaining CBZ and directly accessible Settings.