refactor(launcher): hardcode Gitea channel in lib/baked-gitea-channel.js

- Merge baked base_url/owner/repo/release_tag at load time (no inject script,
  no fractured-release-channel.json, no CI env for pack).
- Fix mergeConfig deep-merge for gitea, patch_manifest, launcher_updates_from_github.
- Remove inject-release-channel.js and fractured-release-channel.json.

Co-authored-by: Cursor <cursoragent@cursor.com>

.github/workflows/fractured-launcher-ci.yml

@@ -38,10 +38,6 @@ jobs:
           cache-dependency-path: tools/fractured-launcher-electron/package-lock.json
 
       - name: Install and pack (NSIS + portable)
-        env:
-          GITEA_BASE_URL: ${{ secrets.GITEA_BASE_URL }}
-          GITEA_OWNER: ${{ secrets.GITEA_OWNER }}
-          GITEA_REPO: ${{ secrets.GITEA_REPO }}
         run: |
           npm ci
           npm run pack:win