Refine reader navigation with separate translation and passage pickers

[Llewellynvdm]

The combined navigation dropdowns made ordinary book/chapter changes cumbersome and mixed them with infrequent translation and appearance choices. This implements the maintainer's bot mini-app navigation pattern across the shared Flutter reader (Android, iOS/iPadOS, web, Windows, macOS and Linux).

• Tap the passage heading for a compact searchable book grid, then choose a published chapter. Extended books, introduction nodes and sparse numbering remain available.
• Change translations separately: search by name/language/abbreviation, inspect the selected translation's details, then explicitly open it. Keep the current book/chapter where available; partial Bibles ask for an available passage.
• Move search and chapter controls toward the left. Appearance, sizing, backups and downloads remain in Reader options; Reference preview has its own heading icon.
• Keep browsing/cancellation read-only, preserve unsaved-note guards and reject results after dismissal. No other Bible is downloaded while previewing candidates.
• Use the same native widgets in compact keyboard-aware sheets and desktop dialogs. Fix a confirmed short-landscape overflow while preserving inline-note keyboard resizing.

Validation on the final implementation:

• 542 Flutter tests passed, including 23 standalone picker checks, 7 composed navigation cases, and the shared native journey's new selector checkpoints.
• Flutter analyzer clean; 258 Dart files formatted; 444-message locale inventory and fixture/branding checks passed.
• 83 developer-tool tests, 64 release-tool tests and 6 browser-harness tests passed.
• Production Web compilation, both resource workers and the validated offline shell passed locally.
• Actual widget captures reviewed for phone/desktop, RTL, 200% text and short keyboard layouts.

The delayed Search Open regression now checks immediate request invalidation and accepts only the new query after its legitimate debounce. This fixes a timing-sensitive assertion without changing search behavior.

Hosted Flutter CI run #80 passed on final head 22b758f38073938378e0fb4db4eac71ba0d25cb7: all 15 applicable jobs succeeded. This includes Android APK/AAB, unsigned iOS device/simulator, Web, Windows, macOS and Linux packages; native acceptance on Android phone/tablet, iPhone/iPad, Windows, macOS and Linux; and all six Chromium/Firefox/WebKit journeys across standard and isolated hosting. The browser harness uses the new accessible passage heading and retains its persistence/offline assertions. Five signing-related jobs were skipped as configured. Physical-device touch, VoiceOver/TalkBack, human language review and signing/store distribution remain separate gates. Details are in docs/TESTING.md. The React reference was inspected at 555008554d46e174dfdfb41e3bdc578d2a62a66e; this PR changes only the requested Flutter app repository.