Cloud-first UX: default to Deepgram, gate start button, add room sharing

- Change default transcription mode from local to byok (cloud/Deepgram)
- Move Transcription Mode selector to top of settings for visibility
- Hide local-only settings (model, VAD, timing) when cloud mode selected
- Disable Start button until API key (byok) or login (managed) is configured
- Add room creation and share code flow to Shared Captions section
- Add POST /api/create-room endpoint to Node.js sync server
- Update default sync URL placeholder to caption.shadowdao.com

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

src/lib/components/Controls.svelte

@@ -1,5 +1,6 @@
 <script lang="ts">
   import { backendStore } from "$lib/stores/backend";
+  import { configStore } from "$lib/stores/config";
   import { transcriptionStore } from "$lib/stores/transcriptions";
 
   let isTranscribing = $derived(backendStore.appState === "transcribing");
@@ -8,6 +9,16 @@
   );
   let isLoading = $state(false);
 
+  let remoteMode = $derived(configStore.config.remote.mode);
+  let byokApiKey = $derived(configStore.config.remote.byok_api_key);
+  let authToken = $derived(configStore.config.remote.auth_token);
+
+  let cloudConfigured = $derived(
+    remoteMode === "local" ||
+    (remoteMode === "byok" && byokApiKey.trim() !== "") ||
+    (remoteMode === "managed" && authToken.trim() !== "")
+  );
+
   let errorMessage = $state("");
 
   async function toggleTranscription() {
@@ -94,7 +105,7 @@
   <button
     class={isTranscribing ? "danger" : "primary"}
     onclick={toggleTranscription}
-    disabled={!isReady || isLoading}
+    disabled={!isReady || isLoading || !cloudConfigured}
   >
     {#if isLoading}
       ...
@@ -116,6 +127,18 @@
   {#if errorMessage}
     <span class="error-msg">{errorMessage}</span>
   {/if}
+
+  {#if !cloudConfigured && isReady}
+    <div class="cloud-warning">
+      {#if remoteMode === "byok"}
+        <span>API key required. Get one at
+          <a href="https://console.deepgram.com" target="_blank" rel="noopener">console.deepgram.com</a>,
+          then enter it in Settings.</span>
+      {:else if remoteMode === "managed"}
+        <span>Login required. Open Settings to log in.</span>
+      {/if}
+    </div>
+  {/if}
 </div>
 
 <style>
@@ -125,6 +148,18 @@
     margin-left: 8px;
   }
 
+  .cloud-warning {
+    font-size: 12px;
+    color: #ff9800;
+    margin-left: 8px;
+    flex: 1;
+  }
+
+  .cloud-warning a {
+    color: #4fc3f7;
+    text-decoration: underline;
+  }
+
   .controls {
     display: flex;
     align-items: center;