Release v1.2.3: simpler update checks

[ixiehao]

Summary

• Check for updates once about 60 seconds after each app launch, including the first; keep the Settings opt-out and Help manual check.
• Remove the idle footer check link. Show an update link only when a new version is found, and hide it when that version is dismissed.
• Keep manual checking available after an update is found and update the banner immediately when the app language changes.
• Bump to v1.2.3/build 9; update three-language strings, changelog, and privacy disclosures.

Validation

• Local swift test: 56 tests, 0 failures, 1 opt-in benchmark skipped.
• ./Scripts/run-media-regression.sh: 28 checks passed.
• Localizations and release build passed.
• Universal DMG mounted, signature and SHA-256 verified. Isolated copy of packaged app: 60-second update discovery, dismissal, Help manual check, Settings text, and synthetic MP4 conversion verified.

The package is ad-hoc signed and not notarized; no Developer ID certificate is required by this project workflow. Two unrelated pre-existing avatar deletions were excluded from this PR.