Fix native streamer fullscreen monitor placement

[capy-ai]

Summary

• keep the Windows external native renderer guard alive for the full stream session so fullscreen/window changes continue to apply
• derive the external renderer target from the Electron render surface HWND and client rect instead of the renderer window's startup monitor
• use that target rect for renderer placement and cursor clipping, allowing fullscreen toggles and secondary-monitor placement to follow the app window

Fixes #497

Verification

• cargo check --manifest-path native/opennow-streamer/Cargo.toml
• cargo check --manifest-path native/opennow-streamer/Cargo.toml --features gstreamer
• cargo test --manifest-path native/opennow-streamer/Cargo.toml
• cargo test --manifest-path native/opennow-streamer/Cargo.toml --features gstreamer

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Recompute the target rect instead of caching screen coordinates

When the Electron BrowserWindow is dragged to another monitor without being resized, the renderer only publishes surface updates for resize/fullscreen/visibility/visualViewport changes (StreamView.tsx), so no new update is sent on a pure window move. This converts the HWND-relative client rect into screen coordinates once and caches only that Rect; the guard then keeps using stale screen coordinates for SetWindowPos and ClipCursor, leaving the external renderer on the old monitor until some later resize/fullscreen event forces another surface update. Store the HWND plus client rect and recompute the screen rect in the guard loop, or have the main process publish move events.

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