SF-37: Add overlay mode capability states

[alexanderbrunker-star]

Summary

• add a shared browser overlay source model and popup UI for tab, macOS, iOS, and Android capability states
• expose iOS and Android Overlay Mode status in native app state/UI
• relabel macOS system capture as Overlay Mode while keeping file fallback and permission messaging explicit
• update README platform capability notes

Verification

• cd sonicflow_app/chrome-extension && npm test -- popup.test.js
• make test-ios
• cd sonicflow_app/android-app && ./gradlew --console=plain :app:testDebugUnitTest --tests 'com.sonicflow.app.ui.FlowTonesViewModelTest'\n- make mac\n\nLinear: SF-37

[linear]

SF-37 SonicFlow overlay mode for Spotify, YouTube, browser tabs, and system audio

Goal

Make external-audio overlay a first-class SonicFlow mode: layer the functional modulation track over Spotify, YouTube, Apple Music, podcasts, local files, or browser tabs where each platform permits it.

Scope

• Define shared Overlay Mode product model and platform capability matrix.
• Chrome/web: prioritize YouTube and supported browser tabs via existing content-script/tab-audio path.
• macOS: use system capture where permitted and retain local file fallback.
• iOS: avoid promising full Spotify/YouTube system capture unless a compliant API path exists; support app-owned/local media first.
• Android: investigate MediaProjection/audio capture policy and implement only legal/reliable sources.
• Show honest source states: connected, unavailable, permission needed, browser-tab only.

Acceptance

• Overlay Mode is explicit in the UI.
• Unsupported sources fail gracefully with clear copy.
• Browser and macOS can mix SonicFlow layer under external audio without hijacking playback.
• Tests cover source selection, permission states, and session continuity when availability changes.