Web: image thumbnails (no full download) + large-folder memoization

[ehsan6sha]

What

Lazy image thumbnails in the web file grids (category tabs + Cloud Files) without downloading whole files, plus a large-folder perf quick-win.

How

• ~10 KB JPEG generated client-side at upload (ui codec downscale → image pkg JPEG), stored as an encrypted sidecar in a parallel <bucket>-thumbs bucket — the grid fetches ~10 KB, never the full file.
  • Why a separate bucket: listObjects loads the whole bucket forest; same-bucket sidecars would double that load and risk the 20 k listing-cache cap — worsening large folders. The sibling bucket keeps the main listing at N; its thumb forest loads once, lazily.
• Never fetch thousands: WebThumbnailService = in-memory LRU + encrypted IndexedDB + a concurrency-capped (5) queue + in-flight de-dup + negative cache; the fetch is 10 s-bounded and a missing -thumbs bucket is marked absent (no doomed per-image burst on a pre-feature bucket). WebThumb widget: instant on a cache hit, else a 250 ms debounce so fast-scrolled-past rows never fetch; cancels on dispose. A miss → icon (never a full download).
• Existing files: backfilled on view (bytes already downloaded), gated to the user's own writable bucket. Thumb follows on rename/move, removed on delete. *-thumbs hidden from the bucket list. Caches cleared on sign-out (clearAll/deactivate).
• Images only (videos/docs keep icons).
• Perf: memoize deriveCloudFolderView (was re-scanning the whole flat list every build + every rename-dialog keystroke).

Verification

• flutter analyze clean across all touched files; flutter build web --release green. The folder-tree unit tests still pass; the thumbnail service is web-only (JS-interop crypto), so its concurrency/cache logic isn't VM-unit-testable — reviewed by the advisor instead (semaphore verified correct), and the two gaps it flagged are fixed: a 10 s fetch timeout, a bucket-level negative cache, and sign-out cache clearing.

⚠️ Live-verify (web, signed in)

1. Upload a new image → row shows a thumbnail; DevTools Network shows a small (~10 KB) …-thumbs fetch, not the full file.
2. Open a large image folder + scroll fast → only a handful of thumb requests fire (≤5 concurrent; scrolled-past rows don't fetch); scroll back → instant.
3. A pre-existing image → icon until opened once; after opening, its thumbnail appears.
4. Rename / move / delete → thumbnail follows / disappears; no *-thumbs bucket appears in the Cloud Files bucket list.

Bumps web version to v1.11.4.17.

🤖 Generated with Claude Code