Add continuous scrolling for multi-page documents

[aljazceru]

Summary

• render pages in a continuous scrolling container and lazily render the visible window
• track the most-visible page and keep late-page jumps pending until their wrapper exists
• coalesce pinch render requests so large documents remain responsive
• add fit-mode and continuous-scrolling controls

Why

Large documents exposed two races in the continuous renderer. Repeated pinch events could queue enough WebView/JavaScript work to delay state reads, and a late-page jump could run before progressive page setup created the target wrapper. The scroll handler could then overwrite the requested page.

Zoom rendering now keeps one request in flight and coalesces pending updates. Page requests remain sticky until the target is laid out. Rendering and relayout are limited to the visible page window.

Validation

• npm test -- --reporter=dot — 14 tests passed
• npx eslint .
• ./gradlew :app:lintDebug :app:assembleDebug :app:assembleDebugAndroidTest --no-daemon
• ANDROID_SERIAL=5A011JEBF06589 ./gradlew connectedDebugAndroidTest --no-daemon — 77 tests passed on a Pixel 9a
• git diff --check

[rdevshp]

I tested your PR using app/src/androidTest/assets/test-multipage.pdf . There are panning issues when zoomed in when the viewer is at page 4 of the test pdf file.