feat(android): remember reading position

[jiasheny]

Summary

• persist normalized reading progress per Android document in a dedicated SharedPreferences store
• keep the source document key on cached Recent entries so reopening either path resumes at the same place
• debounce scroll saves, flush before backgrounding/document switches, and restore after deferred renderer/image layout changes
• add Playwright regression coverage and update Android/mobile documentation

Verification

• node mobile/scripts/verify-mobile-renderer.mjs — passed (repeated 3 times)
• gradle :app:assembleDebug --no-daemon — passed
• git diff --check — passed
• gradle :app:lintDebug --no-daemon — reaches lint, then reports the existing AppLinkUrlError for the unchanged file:// intent filter at AndroidManifest.xml:56; no new reading-position lint error

Covered scenarios

1. Reopening the same long document restores its saved normalized position.
2. Cached Recent entries reuse the source document position.
3. Short/non-scrollable documents clamp safely to the top.
4. Deferred content growth preserves the target progress until the reader interacts.