Show transcription start errors in UI, improve error logging

Start Transcription button now shows the error message when it fails
instead of silently reverting. Common causes:
- Missing PortAudio library on Linux
- Audio device not accessible
- Deepgram connection failure

Also added error details to backend console output and captured
the last error from the Deepgram engine for better diagnostics.

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

src/lib/components/Controls.svelte

@@ -8,9 +8,12 @@
   );
   let isLoading = $state(false);
 
+  let errorMessage = $state("");
+
   async function toggleTranscription() {
     if (isLoading) return;
     isLoading = true;
+    errorMessage = "";
     try {
       if (isTranscribing) {
         await backendStore.apiPost("/api/stop");
@@ -20,8 +23,10 @@
       // Poll status to update UI immediately instead of waiting
       // for WebSocket broadcast (which can be delayed or missed)
       await backendStore.pollStatus();
-    } catch (err) {
-      console.error("Failed to toggle transcription:", err);
+    } catch (err: unknown) {
+      const msg = err instanceof Error ? err.message : String(err);
+      console.error("Failed to toggle transcription:", msg);
+      errorMessage = msg;
     } finally {
       isLoading = false;
     }
@@ -104,9 +109,19 @@
   <button onclick={saveTranscriptions} disabled={!backendStore.connected}>
     Save
   </button>
+
+  {#if errorMessage}
+    <span class="error-msg">{errorMessage}</span>
+  {/if}
 </div>
 
 <style>
+  .error-msg {
+    color: #f44336;
+    font-size: 12px;
+    margin-left: 8px;
+  }
+
   .controls {
     display: flex;
     align-items: center;