bug(frontend): duplicate auto-save on mount — controller and ChatInterface both register independent save intervals

[mrveiss]

Summary

`ChatInterface.vue` registers two independent auto-save intervals on every mount, causing every session to be saved twice per cycle.

Root Cause

`onMounted` in `ChatInterface.vue:822-829` calls both:

```ts
// Call 1 — ChatController interval (30s default, leaked — see related issue)
if (store.settings.autoSave) {
controller.enableAutoSave()
}

// Call 2 — local Vue interval (120s, properly managed)
enableAutoSave()
```

Both check `store.settings.autoSave && store.currentSessionId` and both call `controller.saveChatSession()`. When they fire at the same time, two identical `POST /api/chats/{id}/save` requests hit the backend simultaneously.

Evidence

Backend log for session `82feb428-c4fa-4e4d-ab89-f94c7ca056b2` shows two simultaneous saves at every interval tick:

```
2026-04-07 13:24:59 POST /api/chats/.../save 200 OK
2026-04-07 13:24:59 POST /api/chats/.../save 200 OK ← duplicate same second
2026-04-07 13:25:59 POST /api/chats/.../save 200 OK
2026-04-07 13:25:59 POST /api/chats/.../save 200 OK ← duplicate same second
```

Fix

Remove the `controller.enableAutoSave()` call from `onMounted`. The local `enableAutoSave()` (120s, properly managed with `clearInterval` guard) is sufficient. The controller method is redundant and also leaks its interval (see related issue).

[mrveiss]

The root cause — call in — was removed as part of the fix for #3749 in PR #3760. This issue is fully resolved by that change.