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>

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,