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/backend.svelte.ts

@@ -255,6 +255,14 @@ export const backendStore = {
   get wsUrl() {
     return `ws://localhost:${state.port}/ws/control`;
   },
+  get obsUrl() {
+    // OBS display runs on the web server port (one below the API port)
+    const obsPort = state.port > 0 ? state.port - 1 : 8080;
+    return `http://localhost:${obsPort}`;
+  },
+  get syncUrl() {
+    return "";
+  },
   setPort,
   connect: connectWebSocket,
   disconnect,

vite.config.ts

@@ -10,6 +10,7 @@ export default defineConfig({
     alias: {
       $lib: path.resolve("./src/lib"),
     },
+    extensions: [".svelte.ts", ".ts", ".svelte", ".js", ".mjs", ".mts"],
   },
   server: {
     port: 1420,