PUB-8: Add automatic app update checks

[rovkinmax]

Task Summary

• Adds automatic GitHub latest-release checking for Puber releases from rovkinmax/Puber.
• Prompts the user to update when a newer APK release is available, downloads the APK, verifies optional SHA-256 checksum assets, and opens Android's package installer.
• Adds a local default-enabled setting to disable automatic update checks.

Compliance Review

• Mandatory Compliance Review passed for PUB-8 automatic-version-check.
• No compliance violations were found in the reviewed scope.

Verification

• Passed: ./tools/agentw :app:compileDevDebugKotlin.
• Passed: ./tools/agentw :app:testDevDebugUnitTest --tests '*AppVersionTest' --tests '*AppUpdate*Test' --tests '*UpdatePromptVMTest'.
• Passed focused emulator smoke with fresh devDebug APK install: app launched, GitHub latest-release request returned HTTP 200 without KinoPub Authorization on the GitHub request, update setting was visible/focusable/toggled/restored, and no fatal crash was observed.

Known Skipped Checks / Limitations

• Smoke did not force a synthetic newer release or complete an APK installer E2E flow.