feat(updates): browse and download historical channel builds
Add a paginated version selector for Stable, Preview and Experimental, including
commit titles, source SHAs and available commit descriptions. Manual browsing
bypasses newer/ignored-version gates while retaining APK flavor and ABI matching.
Use one persistent sheet state for loading, selection, download progress, errors
and installation. Pin active downloads, prevent duplicate writers and dismissals,
retain resumable files, verify cached APKs, and handle installer permission errors.
Archive signed CI APKs as complete prereleases with permanent download URLs before
rolling Pages feeds replace them. Keep previous builds and existing stable/latest
release behavior. Add all new text to existing strings.xml files in all 11 locales.
Validation: static source review, resource XML/reference checks, workflow YAML and
Python/shell syntax checks, and git diff --check. No local builds, compilation or
tests were run or generated. Normal GitHub CI is enabled.