perf(photos): append-only render eliminates DOM rebuild on scroll

Replace the innerHTML full-rebuild in _render() with two paths:
- _renderFull(): used for first load, group-mode change, and deletions
- _appendBatch(n): append-only for infinite-scroll pages — O(batch)
  instead of O(total). Existing <img> nodes are never destroyed,
  eliminating the visual flash and unnecessary DOM churn.

Also:
- Extract _renderTile() helper (DRY tile HTML generation)
- Extract _observeSentinel() helper
- Scope _setupVideoThumbnails(startIndex) to only process new tiles
- Add data-group attribute on headers for efficient CSS.escape lookup
- Fix stale WebP references in comments (now JPEG)
- Add virtual scrolling idea to TODO-LIST.md for future evaluation

TODO-LIST.md

@@ -34,6 +34,10 @@ This document contains the task list for the development of OxiCloud, a minimali
 - [x] Implement multiple file uploads
 - [ ] Add progress indicators for long operations
 - [x] Implement UI notifications for events
+- [ ] Photos timeline: virtual scrolling
+  - Solo mantener en el DOM las filas visibles en el viewport + un margen. Al hacer scroll, reciclar los nodos que salen por arriba para los que entran por abajo.
+  - Ventajas: Funciona perfectamente con 50,000 fotos. Uso de memoria constante.
+  - Excesiva para ahora — evaluar cuando el volumen de fotos lo justifique.
 
 ## Phase 2: Authentication and Multi-User