Hide Local (Whisper) mode option when using cloud-only sidecar

- Expose is_cloud_only flag in /api/status response
- Add isCloudOnly to backend store state
- Conditionally hide Local (Whisper) radio button in Settings

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/lib/components/Settings.svelte

@@ -453,6 +453,7 @@
             />
             Managed Service
           </label>
+          {#if !backendStore.isCloudOnly}
           <label>
             <input
               type="radio"
@@ -462,6 +463,7 @@
             />
             Local (Whisper)
           </label>
+          {/if}
         </div>
         {#if remoteMode === "byok"}
           <div class="field">