Live camera source for Scan — and wrestling Swift 6 strict concurrency

The next step in the "swap the image source" plan: PhotosPicker → phone camera → (eventually) Meta glasses feed. Each is just a different way to produce image Data; the Vision OCR → /api/translate → display pipeline is identical regardless.

What landed (commit 9272b69)

Camera.swift adds CameraModel (an AVCaptureSession + photo output) and CameraCaptureView (a live preview with a shutter button). Capturing a photo resolves to Data, which flows into the same process(data) that PhotosPicker already used. ScanView now shows two source buttons — Camera (live) and Photo (picker) — so it works both on a real device and in the simulator.

The actual work: Swift 6 strict concurrency

SWIFT_STRICT_CONCURRENCY: complete and AVFoundation don't get along by default. The build failed twice:

• AVCaptureSession is non-Sendable, so capturing it into a Task.detached (to run the blocking startRunning() off the main actor) is a data-race error.
• The capture-photo delegate callback is nonisolated (arbitrary queue), but it needs to resume a @MainActor continuation.
• A @MainActor View method returning some View couldn't be called from PhotosPicker's nonisolated label closure.

Fixes (recorded in docs/troubleshooting.md):

• An @unchecked Sendable SessionBox carries the session across to the background Task; the delegate maps everything to a Sendable Result<Data, CameraError> before hopping back to @MainActor.
• The shared label became a SourceButtonLabel: View struct — constructed, not a MainActor method call — so nonisolated closures can build it.

After those, BUILD SUCCEEDED with zero concurrency warnings.

Verified — and not

• xcodebuild → BUILD SUCCEEDED (iphonesimulator), Swift 6 strict concurrency clean.
• App smoke-launched in the iPhone 17 Pro simulator (no startup regression from the camera code).
• Not verified: the camera itself — the simulator has no camera, so live capture only runs on a real device (my iPhone 16 Pro Max). The Photo source covers the OCR→translate path in the simulator. Translation output still needs valid API keys.

This keeps the architecture honest: the camera is just another Data source. When the Meta glasses path opens up, the MWDAT camera feed slots into the exact same process(data) — no pipeline rewrite.

Commit: 9272b6917d0519f9f00f8ac1d7c5f1f7d9cee17c