* allow users to select live streaming technology
* fix webrtc being downgraded to mse on load
`useUserPersistence` seeds state with the default and loads asynchronously, so the first render always used `mse` instead of the saved choice, and `useWebRTCGloballyAvailable` reports `checking` until the probe settles and re-enters that state on every consumer mount, so a saved `webrtc` was rewritten to `mse` even after the probe had already passed. On Safari the MSE player then timed out and latched the jsmpeg fallback. A pending probe now counts as available, the player waits on `autoLive` until the stored preferences load, and `handleError` gates on the mode in use since the fallback flag no longer implies webrtc is untried. A rejected IndexedDB read also resolves `loaded` now, so a blocked store can't leave the player waiting forever.
* add support for configurable ICE servers in WebRTC player
* add mic error state, fix dialog overwriting saved choice and dashboard ignoring stream
* tweaks
* add import/export for camera group layouts and streaming settings
Camera group layouts and per-camera streaming settings are stored in the browser's IndexedDB, so they are tied to a single browser on a single device. Users with more than one device have to rebuild every group layout and re-pick every camera's stream settings by hand, and clearing browser data loses the work.
Add a Backup & Restore card to Settings > UI Settings that exports these settings to a JSON file and imports that file on another device. Import shows a confirmation dialog with per-section counts, switches for layouts, streaming settings, and UI preferences, and warnings about camera groups or cameras in the file that are not on this server.
Server-side storage is deliberately avoided. These are per-device presentation settings: a layout arranged for a desktop is wrong on a tablet, and continuous full-resolution streams that are free on a wired LAN are not on a phone. An explicit file moves settings only when the user chooses to move them.
Implementation notes:
- web/src/utils/uiSettingsTransfer.ts owns a registry of transferable IndexedDB keys. Each entry records whether the key is user-namespaced, matching which persistence hook wrote it, plus a zod schema for its value.
- Only registry-known keys are ever written, and only when their value passes that schema. The file format deliberately lets unknown keys survive parsing, so this filter is what prevents a hand-edited file from writing arbitrary storage keys or out-of-range values.
- Export falls back to the legacy un-namespaced key, because the username migration runs lazily on first mount of each owning hook.
- Streaming settings merge per group rather than replacing the whole map, so groups configured only on the receiving device survive.
- Import writes storage and then reloads, because useUserPersistence reads a key only on mount and StreamingSettingsProvider would otherwise write its stale in-memory state back over the import.
- playbackBandwidthEstimate, frigate-search-history, and live-layout are excluded: the first two are measurements and user data rather than preferences, and live-layout's default is derived from the device.
* merge imported streaming settings per camera instead of per group
