Add iOS screen sharing to Gemini realtime calls

[yagudaev]

Summary

• New expo-screen-capture native module (iOS) using RPScreenRecorder for in-app capture and a ReplayKit Broadcast Upload Extension (BroadcastUpload.appex) for background capture, communicating with the host app through an App Group container + Darwin notifications.
• Added sendFrame(base64Jpeg) to useRealtime, forwarding JPEGs to the existing relay frame.append path (no relay/server changes needed).
• Call UI: new Screen-Share toggle button in realtime calls, gated on Gemini models only. Tap = in-app capture; long-press = presents the system broadcast picker for background capture.
• Frames throttled to 1 FPS, scaled to 768px max, JPEG quality 0.7 (matches the desktop reference implementation).
• New config plugin with-broadcast-extension programmatically registers the extension target in the Xcode project and adds the group.com.yagudaev.voiceclaw.broadcast App Group to the main app's entitlements.

Files of note

• mobile/modules/expo-screen-capture/ — Swift native module + TS bindings
• mobile/broadcast-extension/SampleHandler.swift — ReplayKit sample handler (1 FPS, JPEG → App Group file → Darwin notification)
• mobile/plugins/with-broadcast-extension.js — Expo config plugin; patches pbxproj with fixed UUIDs for idempotency
• mobile/lib/use-realtime.ts — new sendFrame method on the realtime controls
• mobile/app/(tabs)/index.tsx — UI toggle, event subscriptions, teardown on call-end

Test plan

• BroadcastUpload target compiles cleanly on iphonesimulator (verified via xcodebuild -target BroadcastUpload)
• Prebuild runs the new config plugin without error; pbxproj additions are idempotent
• Pending: on-device test on iPhone 12 Pro — blocked by provisioning (the group.com.yagudaev.voiceclaw.broadcast App Group needs to be registered in Apple Developer portal and added to the dev provisioning profile before the main app / extension can be signed for device). Full-app simulator build is blocked on a pre-existing swift-numerics modulemap path-resolution issue unrelated to this change.
• Manual: after provisioning is updated, verify tap toggles in-app capture (frames visible in relay logs) and long-press launches the system broadcast picker that keeps streaming when the app backgrounds.

Follow-ups

• Register App Group in Apple Developer portal / regenerate provisioning profiles.
• Consider exposing a distinct UI affordance for background-share vs. in-app-share (currently tap/long-press) once UX is validated.
• No OpenAI path; screen-share button is hidden when the active model isn't a Gemini realtime model (per PRD).

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