CI: make builds work on China-based self-hosted Gitea runner

The self-hosted Windows runner can't reach github.com (GFW), so the previous
workflow failed cloning actions/checkout and would also fail downloading the
Flutter SDK and the sqlite3 native binary.

- Vendor the official, SHA-256-verified sqlite3 binaries under vendor/sqlite3/
  and select them via pubspec hooks.user_defines (source: test-sqlite3). Builds
  and tests now run fully offline — no GitHub download, no proxy, no
  LD_LIBRARY_PATH hack (removed .local-sqlite/).
- Consolidate CI into one Windows workflow: fetch actions from the gitea.com
  mirror, use the runner's pre-installed Flutter (no SDK download), and use the
  flutter-io.cn pub/Flutter mirrors. Server tests use the Tsinghua PyPI mirror.
- Document the offline build + China mirrors in README.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

.gitignore

@@ -47,9 +47,6 @@ app.*.map.json
 # OMC orchestration state
 .omc/
 
-# Local-only sqlite3 override for offline/firewalled test runs
-.local-sqlite/
-
 # Python server artifacts
 server/.venv/
 server/.omc/