Add fullscreen landing image viewer

[VeldtJumper]

Summary

• make all three landing-page screenshots tappable in compact and wide layouts
• open the selected screenshot in a dedicated fullscreen Navigation3 destination with an animated scrim, fitted image, safe-area-aware close control, and accessibility labels
• serialize the selected screenshot and give each viewer route a unique navigation identity
• reconcile browser Back and Forward history with route tokens, root-generation invalidation, and exact restoration for consumed multi-step Back events
• add tests for viewer selection/serialization and browser-history behavior

Verification

• ./gradlew :app:shared:testAndroidHostTest :app:androidApp:assembleDebug :app:androidApp:assembleRelease :app:androidApp:lint
• browser-history bridge JavaScript syntax check and mocked Back/Forward scenarios
• git diff --check
• independent closure review: no findings

Notes

• JS/Wasm/web Gradle targets were intentionally left to CI per the repository's local verification policy.
• Manual interaction testing was unavailable because no Android device or emulator was attached.