Open element photos full screen with pinch-to-zoom

[dfalling]

Summary

Photos on the element detail screen were only ever visible as 200×140 thumbnails. They're now tappable and open a full-screen viewer:

• Pinch to zoom (1×–4×), anchored on the fingers so the photo doesn't drift while zooming
• Drag to pan while zoomed, clamped to the photo's edges
• Double tap to toggle 2.5×, centered on the tapped point
• Swipe the photo away in any direction to close — it tracks the finger and fades, with a fling threshold so a quick flick counts
• × button at the top-left, plus hardware back

Notes

• Gestures are hand-rolled on PanResponder + Animated rather than adding react-native-gesture-handler + react-native-reanimated. ScrollView's pinch-zoom props are iOS-only, so there was no built-in path, and two native modules for one screen's zoom seemed like the worse trade. The transform geometry lives in src/photos/photoZoom.ts as pure functions (same split as placeZoom.ts) with 12 tests; the component only translates touches into it. If the dependency is preferred later, the component is the only thing that changes.
• The viewer is an absolute overlay, not a Modal, for the reason already documented in Sheet.tsx: Modal gets its own window and doesn't reliably draw under Android 15's system bars, so a "full-screen" viewer would stop short of them.
• It renders photo.regular — the same URL as the thumbnail — so opening it fetches nothing new. Photo also has a full field that would look sharper at 3–4×, but wiring it up needs a codegen run against the uncommitted schema plus the Apollo v4 fixups; left for a follow-up.
• Black in both themes rather than themed, on the grounds that a photo viewer should let the image set the mood. Commented in the file.

Testing

bunx tsc --noEmit, bun run lint, and bun run test all pass.

Not run on a device — no emulator or handset was attached — so the gesture feel (spring stiffness, dismiss threshold, double-tap window) is untested in the hand and may want tuning.