Harden Android screen-share startup

[phucnguyen1707]

Summary

• keep Android screen sharing and WebRTC hosting alive while notification and MediaProjection system prompts temporarily background the app
• request Android 13+ notification permission on a best-effort basis without blocking screen-share startup
• declare the notification permission and add an installable Android preview APK profile
• validate the generated native project in CI for the exact screen-share permission set and MediaProjection service wiring

Safety

A genuine background transition still stops capture and hosting. Only transitions that occur while a known native prompt is active are deferred, and the deferred state is reconciled when the prompt finishes.

Validation

• mobile lint and typecheck: pass
• mobile tests: 14 files, 125 passed
• Android bundle export: pass
• clean Android prebuild: pass
• generated native screen-share verifier: pass
• final Fable max-effort review: approved

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