chore: drop epubjs

Nothing imports it since the foliate rewrite. Also note the one-shot purge of
its locations cache in TODO.md, so the temporary code gets removed later.

TODO.md

@@ -51,6 +51,20 @@ Worth adding at the same time:
 
 ## Frontend
 
+### Remove the epub.js locations-cache purge
+
+`frontend/src/lib/reader/legacy-cache.ts` — `purgeLegacyLocationCache`
+
+The epub.js reader cached generated locations in `localStorage` under
+`${bookId}-locations`, a few hundred KB of JSON per long book. foliate computes
+progress from section byte sizes at open time, so nothing writes those keys any
+more, but existing browsers still hold them — and a reader near the 5–10 MB
+origin quota would make the new reader-settings write throw `QuotaExceededError`.
+
+The reader clears them once per browser, behind a `chitai:locations-purged` flag.
+Delete the module, its call in `epub-reader.svelte` and the flag once deployments
+have had a release or two to run it — after roughly 2026-12.
+
 ### Cover dimensions are unknown until load
 
 `book-cover.svelte` renders covers at a fixed height with natural width so nothing is