Fix blank screen: rename stores to .svelte.ts for rune support

Svelte 5 runes ($state, $derived, $effect) are only compiled in
.svelte and .svelte.ts files. The stores used runes in plain .ts
files, which meant $state was treated as an undefined function at
runtime, crashing the JS before anything rendered.

- Renamed backend.ts -> backend.svelte.ts
- Renamed config.ts -> config.svelte.ts
- Renamed transcriptions.ts -> transcriptions.svelte.ts
- Added .svelte.ts to Vite resolve extensions
- Added missing obsUrl/syncUrl getters to backend store

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

src/lib/stores/transcriptions.svelte.ts

@@ -0,0 +1,109 @@
+/**
+ * Transcriptions store - manages the list of transcription items
+ * received from the backend via WebSocket.
+ */
+
+export interface TranscriptionItem {
+  id: string;
+  text: string;
+  userName: string;
+  timestamp: string;
+  isPreview: boolean;
+}
+
+let items = $state<TranscriptionItem[]>([]);
+let nextId = 0;
+
+function generateId(): string {
+  return `t-${Date.now()}-${nextId++}`;
+}
+
+function addTranscription(data: {
+  text?: string;
+  user_name?: string;
+  timestamp?: string;
+}) {
+  // When a final transcription arrives, remove any existing preview
+  const previewIndex = items.findIndex((item) => item.isPreview);
+  if (previewIndex !== -1) {
+    items.splice(previewIndex, 1);
+  }
+
+  items.push({
+    id: generateId(),
+    text: data.text ?? "",
+    userName: data.user_name ?? "",
+    timestamp: data.timestamp ?? "",
+    isPreview: false,
+  });
+
+  // Keep a reasonable limit
+  if (items.length > 500) {
+    items.splice(0, items.length - 500);
+  }
+}
+
+function setPreview(data: {
+  text?: string;
+  user_name?: string;
+  timestamp?: string;
+}) {
+  const existingIndex = items.findIndex((item) => item.isPreview);
+  const previewItem: TranscriptionItem = {
+    id: existingIndex !== -1 ? items[existingIndex].id : generateId(),
+    text: data.text ?? "",
+    userName: data.user_name ?? "",
+    timestamp: data.timestamp ?? "",
+    isPreview: true,
+  };
+
+  if (existingIndex !== -1) {
+    items[existingIndex] = previewItem;
+  } else {
+    items.push(previewItem);
+  }
+}
+
+function clearAll() {
+  items.length = 0;
+}
+
+function getPlainText(): string {
+  return items
+    .filter((item) => !item.isPreview)
+    .map((item) => {
+      let line = "";
+      if (item.timestamp) line += `[${item.timestamp}] `;
+      if (item.userName) line += `${item.userName}: `;
+      line += item.text;
+      return line;
+    })
+    .join("\n");
+}
+
+// Listen for backend events
+if (typeof window !== "undefined") {
+  window.addEventListener("backend:transcription", ((e: CustomEvent) => {
+    addTranscription(e.detail);
+  }) as EventListener);
+
+  window.addEventListener("backend:preview", ((e: CustomEvent) => {
+    setPreview(e.detail);
+  }) as EventListener);
+}
+
+export const transcriptionStore = {
+  get items() {
+    return items;
+  },
+  get currentPreview(): TranscriptionItem | null {
+    return items.find((item) => item.isPreview) ?? null;
+  },
+  get transcriptions(): TranscriptionItem[] {
+    return items.filter((item) => !item.isPreview);
+  },
+  addTranscription,
+  setPreview,
+  clearAll,
+  getPlainText,
+};