[PB-6480] feature/Filmstrip scrub and scroll restoration in photo viewer

[CandelR]

Add filmstrip scrub and scroll restoration to the photo preview screen

Summary

• PreviewCarousel: drag on the thumbnail strip now changes the main preview photo in real time. Uses isUserScrollingRef to stay active through the momentum phase so the final settled index is captured correctly. filmstripPadding corrected to center the active thumb (screenWidth/2 - (THUMB_W_ACTIVE/2 + THUMB_ACTIVE_MARGIN)) instead of the inactive slot midpoint.
• PreviewPager: receives isScrubbing prop and reads it via ref (isScrubbingRef) excluded from useEffect deps. Prevents an oscillation loop where scrub-end would trigger an animated scroll, fire onMomentumScrollEnd, and restart the cycle.
• usePreviewSource: full resolution fetch is deferred while scrubbing. Replaced uri: string | null | undefined with uri: string | null + isLoading: boolean so loading state is explicit rather than encoded as undefined.
• PreviewPage: drills isScrubbing to usePreviewSource. Passes isLoading to PageContent replacing the uri === undefined spinner check.
• PhotoPreviewScreen/index: owns isScrubbing stat. Reports the current item id to PhotosScreen via onCurrentItemChange route param on every index change.
• PhotosTimeline: converted to forwardRef exposing scrollToAssetId(id).
• PhotosScreen/index: stores the last viewed photo id in lastViewedIdRef. On useFocusEffect scrolls the grid back to that photo and clears the ref.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud