bugfix: tighten validation, drop dead sendBeacon, NUL byte (0.34.1)

Five small fixes from REVIEW.md §2/§4/§8:

- attach_tag: 64-char cap at the handler so the validation error
  envelope matches username/collection-name.
- create_token: same 64-char cap on bot token names.
- LocalStorage::resolve rejects NUL bytes explicitly so callers see
  BadKey instead of an opaque IO error.
- sendBeacon dropped from the reader's pagehide flush — it's POST-only
  and the server's read-progress route is PUT, so every page-close
  was logging a 405 then falling through to the same keepalive fetch
  anyway. Keepalive fetch is now the only path.
- Frontend logout sets content-type: application/json for symmetry
  with the other mutation helpers.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.34.0",
+  "version": "0.34.1",
   "private": true,
   "type": "module",
   "scripts": {