refactor(frontend): extract useSessionListStreams from App.vue

[attson]

Summary

Seventh slice of milestone 5-a. App.vue carried three module-scope transport handles for the session lists:

```ts
let localSessionListConn: SessionListConnection | null = null;
let remoteSessionListConn: SessionListConnection | null = null;
let remotePollHandle: number | null = null;
```

Every attach/replace had to detach the old handle first, and the remote side had two competing transports (WS + HTTP poll) that had to be mutually exclusive. That policy was open-coded across six sites (`connectLocalSessionList`, `connectRemoteSessionListWS`, `stopRemotePoll`, `startPlatformRemotePoll`, `connectRemoteSessionList`, and the web-boot catch block that fell back to polling on error) — each one manually calling `?.detach()` + `clearInterval` + reassigning the module handle.

Pull the three handles + the transport policy into `composables/useSessionListStreams.ts`, exposing:

```ts
attachLocal(endpoint, callbacks)
attachRemoteWS(endpoint, callbacks) // stops any active poll first
startRemotePoll(fn, intervalMs) // stops any active WS first
stopRemote() // detach WS + clear poll
isRemoteWSAttached() // for startPlatformRemotePoll gate
detachAll() // onUnmounted cleanup
```

The composable is purely the transport layer — all app-level state (list refs, endpoint refs, `missingSince` map, config dedup key, apply / prune functions) stays with the caller. The WS ↔ poll mutex is now enforced by construction: `startRemotePoll` always calls `stopRemote` first, `attachRemoteWS` too.

App.vue side:

• Drop the three module-scope handles.
• Drop the standalone `stopRemotePoll()` helper (was 8 lines).
• Drop the `SessionListConnection` import.
• Every attach/detach site collapses to one composable method call.
• `startPlatformRemotePoll`'s "is WS already up?" check moves from a `remoteSessionListConn` module-var peek to `sessionListStreams.isRemoteWSAttached()`.

Net: App.vue -32 / +15; new composable +108. App.vue now 1749 lines (was 1766).

Test plan

• `npm test` 1631 pass
• `npm run build` (vue-tsc + vite) green
• Manual (desktop): boot with relay configured → `/client-sessions` WS attaches; kill the WS → HTTP poll takes over at 2s; toggle relay off → both stop; toggle back on → WS reattaches
• Manual (web): boot → `refreshPlatformRelayState()` uses WS when the relay's proxy is reachable; else the 3s poll runs; reload with no relay config → no polling, no WS