Show detected updates on app entry and retry transient checks

[andre487]

Previously, a background check could find an update while notifications were blocked and leave no visible reminder when the user opened Message487. Failed checks waited for the next daily run, and the update screen showed no background-check result.

Ports the applicable updater behavior from AndroidMegaProxy #61:

• Persist detected versions and offer an entry/foreground dialog with Update, Remind me in a week and Skip this version. GitHub opens the existing screen for a fresh check and explicit download consent; F-Droid opens its package page. Snooze/skip applies to both dialog and notification, while manual checks remain available.
• Show the latest background attempt's time, source and result without overwriting it during manual checks. Preserve cached detection on errors and reject stale-source or already-installed versions.
• Request exponential JobScheduler retries starting at 30 minutes for network errors and HTTP 429/5xx. Other errors retain the daily schedule; disabling automatic checks or changing source suppresses stale retries.
• Adapt lifecycle handling so entering from an update notification suppresses the duplicate dialog on the first resume, while later foreground entries can still show pending updates. Add EN/RU documentation and regression tests.

Validation: 78 JVM/Compose tests, debug/release lint, debug and unsigned release APK builds, unsigned-signature verification, and git diff --check passed locally. Hosted CI runs the API 26/35 emulator, Python and live n8n suites. Real daily/weekly scheduling, OEM restrictions and installation of a signed update remain device acceptance scenarios.

APK artifacts

• Download debug APK — signed with the disposable Android debug key
• Download unsigned release APK — verified unsigned

Built from commit 03d1e769 by CI run #59.
Artifacts expire after 14 days. Neither APK uses the Message487 release key.