ui: loading states, player failure overlay + retry, toasts, back nav, full history page with delete

Bundle 1-4+7 from PLAN.md: skeleton loaders, player Loading/Buffering/fatal overlay with Retry, action toasts, back-to-home crumb + logo fix, ?history full-history view with per-entry delete (GET /api/history?all=1, DELETE /api/history). 14/14 tests green.

README.md

@@ -41,11 +41,14 @@ the app or CLI are rejected immediately, even with an old cookie.
   PBKDF2-SHA256 hashes in `server/data/users.json` (0600).
 - Per-user watch history (`server/data/history/<user>.json`): position
   saved every ~10s, resume where you left off, top-5 "recently played"
-  on the home page. Nothing stored in the browser — new devices pick
-  up your watch state after login, isolated per account.
-  In-header chip shows the user, with logout + delete-account.
+  on the home page, full history page with per-entry delete. Nothing
+  stored in the browser — new devices pick up your watch state after
+  login, isolated per account. In-header chip shows the user, with logout
+  + delete-account (hamburger menu).
 - Search with poster grid, detail view with synopsis + season tabs
-- HLS playback (vendored hls.js), browser ABR across 1080/720/360p
+- HLS playback (vendored hls.js), browser ABR across 1080/720/360p;
+  loading/buffering overlay, and a failure overlay with Retry when a
+  stream can't be played
 - Dub/sub toggle
 - Fast repeat navigation: upstream data is TTL-cached and HLS
   playlists/segments are LRU-cached, so re-opening a show or re-seeking is