Prepare the v0.2.1 release

[Renakoni]

Summary

Prepare MarkText for Android v0.2.1.

• bump Android versionCode from 2 to 3
• set the Android, package, and About-page version to 0.2.1
• keep this patch release limited to the fixes already merged after v0.2.0

Included changes

• more reliable update checks and refreshed About icons (Make the update check immune to GitHub rate limiting, refresh the About icons #203)
• content-only line counts and selection word counts (Count only content lines and show the selection word count (#199) #204)
• toolbar taps no longer reopen a dismissed keyboard (Stop toolbar taps from resummoning the dismissed keyboard (#200) #206)
• a simpler Open page (Remove the Browse all row from the Open page (#185) #207)
• gentler pointer caret scrolling (Make pointer caret scrolling gentler #208)
• stale selection dismissal can no longer replace a newer selection (Prevent stale selection dismissal from replacing newer selections #209)

Verification

• Web unit tests: 83 files, 705 tests
• Muya gate: 221 files, 1549 tests
• Android JVM tests: passed
• production build: passed
• tracked-source lint: passed

The full local pnpm lint also scans a git-ignored local-archive/ directory on this machine; those archive-only findings are absent from a clean checkout and unrelated to this PR.