v2.1

Add Zotero 10 compatibility

Zotero 10 broke several APIs (singular selection getters, Search
condition changes, Zotero.CookieSandbox, item.setType validation), but
ZotMeta uses none of them, so the blocking issue was the compatibility
metadata: bump strict_max_version to 10.0.* and point the fork's URLs at
this repo.

Alongside that:

- Register menus through Zotero.MenuManager (Zotero 8+), which registers
  once app-wide and cleans up its own DOM on shutdown, falling back to
  the existing per-window DOM injection on Zotero 7. MenuManager only
  accepts l10nID, so the two hardcoded menu labels move into the FTL
  file.
- Drop the Zotero 6 leftovers (install.rdf, chrome.manifest,
  overlay.xul/js, the .properties locale), which have been inert since
  Zotero 7. The Makefile and release workflow now read the version from
  manifest.json.
- Remove ZotMeta.getLibraryStats() and its helpers: nothing called them,
  and they invoked Zotero.Items.getAll() without a libraryID.
  preferences.js has a working async, DB-backed equivalent.
- Cover both menu registration paths in the test suite.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DSmvpUNcBEDGZLDrfgWfCb