MED mobile: Feature-detect tab audio and screenshot capture

[MikeSchirtzinger]

Problem

Tab audio and screenshot capture depend on getDisplayMedia, which is unavailable or limited on many mobile browsers. The current UI can expose controls that will fail at runtime.

Evidence

• apps/web/js/capture.js:184-198 calls navigator.mediaDevices.getDisplayMedia directly for tab/system audio and screen capture.
• index.html:1508-1510 defines the + Tab Audio button without mobile/feature-specific capability gating in the markup.

Acceptance criteria

• Detect navigator.mediaDevices?.getDisplayMedia before showing/enabling tab-audio and screenshot capture controls.
• Provide clear unavailable-state UI when the browser cannot support the feature.
• Ensure unsupported mobile browsers do not throw uncaught errors when users tap related controls.
• Add a browser smoke check for both supported and unsupported capability states.