Huddle mediaDevices access crashes the whole app when navigator.mediaDevices is undefined

## Summary

In a WKWebView running in a non-secure context, `navigator.mediaDevices` is `undefined`. The huddle code accesses it unguarded in mount-time effects, so the entire React tree crashes to the "Something went wrong" error boundary with:

```
undefined is not an object (evaluating 'navigator.mediaDevices.enumerateDevices')
```

## Where (v0.4.26, `a31fc4d2`)

Unguarded, and reached from `useEffect` on mount (i.e. crashes without the user ever starting a huddle):

- `desktop/src/features/huddle/lib/useAudioDevices.ts:20` — `navigator.mediaDevices.enumerateDevices()`, plus `addEventListener` / `removeEventListener` at :30 / :32
- `desktop/src/features/huddle/HuddleContext.tsx:168` / `:173` — `navigator.mediaDevices.addEventListener` / `removeEventListener` (output-device effect)
- `desktop/src/features/huddle/HuddleContext.tsx:374` — `navigator.mediaDevices.getUserMedia` (huddle-join path; crashes on join rather than on mount)

## Prior art in the codebase

The profile avatar path already guards this exact pattern:
`desktop/src/features/profile/lib/animatedAvatarCapture.ts:128` — `if (!navigator.mediaDevices?.enumerateDevices) return [];`

So the huddle paths are just inconsistent with the app's own established convention. (Note `getUserMedia` at `animatedAvatarCapture.ts:154` is likewise unguarded, if you want to standardize across both features.)

## Repro

Run the desktop app in a WKWebView context where `navigator.mediaDevices` is unavailable (e.g. a non-secure-context webview). On mount of the huddle context, the app crashes to the error boundary.

## Suggested fix

Guard the huddle `mediaDevices` accesses the same way the avatar code does — degrade to an empty device list / skip listener registration rather than throw. The mount-time effects should no-op when `navigator.mediaDevices?.enumerateDevices` is absent; the `getUserMedia` join path should surface a clear "microphone unavailable" error instead of the opaque crash. Happy to open a PR.

Start with desktop/src/features/huddle/lib/useAudioDevices.ts and the mediaDevices effects in desktop/src/features/huddle/HuddleContext.tsx, then compare desktop/src/features/profile/lib/animatedAvatarCapture.ts:128. Verify the mount-time effects no-op when mediaDevices is unavailable and the join path reports a clear microphone-unavailable error instead of crashing.

Written by the indexing model from the issue text.