feat(version): derive data cache version from content hash of the data tree

Replace the hand-maintained DATA_VERSION constant with a build-time SHA-256
hash of static/data/ embedded as a 'version' field in the preload manifest.
The runtime reads it via fetchWithFallback and clears the IDB cache when it
differs, so any data edit auto-invalidates cached data — no manual bump to
forget. The no-version (offline/stale) and IDB read-error guards are kept,
so the cache is never wiped on an unreadable signal.

Closes audit item L1.

README.md

@@ -34,7 +34,7 @@ npm run dev
 
 Data files in `static/data/` are tracked in git. The only gitignored output is `scripts/output/minei.json` (33MB Chetyi-Minei source).
 
-**Data version**: `DATA_VERSION` in `src/ui/app.ts` controls cache invalidation. Bump it when restructuring data (new fields, reorganized bundles, updated sources) to force users to re-fetch cached data on next app load.
+**Data version**: derived automatically at build time from a content hash of `static/data/`, embedded as a `version` field in the preload manifest. Any data change (including edits to existing files) auto-invalidates cached data on the next app load — no manual bump required. Regenerate with `npm run generate-manifest` and commit the manifest with the data changes.
 
 **Preload manifest**: After adding or removing data files, regenerate the preload manifest:
 ```bash