feat: @agor/stores package (3 stores) + 58 BackendAdapter tests

@agor/stores:
- theme.svelte.ts, notifications.svelte.ts, health.svelte.ts extracted
- Original files replaced with re-exports (zero consumer changes needed)
- pnpm workspace + Vite/tsconfig aliases configured

BackendAdapter tests (58 new):
- backend-adapter.test.ts: 9 tests (lifecycle, singleton, testing seam)
- tauri-adapter.test.ts: 28 tests (invoke mapping, command names, params)
- electrobun-adapter.test.ts: 21 tests (RPC names, capabilities, stubs)

Total: 523 tests passing (was 465, +58)

vite.config.ts

@@ -7,6 +7,7 @@ export default defineConfig({
   resolve: {
     alias: {
       '@agor/types': path.resolve(__dirname, 'packages/types/index.ts'),
+      '@agor/stores': path.resolve(__dirname, 'packages/stores/index.ts'),
     },
   },
   server: {
@@ -17,6 +18,7 @@ export default defineConfig({
   test: {
     include: [
       'src/**/*.test.ts',
+      'packages/**/*.test.ts',
       'sidecar/**/*.test.ts',
       ...(process.env.AGOR_EDITION === 'pro' ? ['tests/commercial/**/*.test.ts'] : []),
     ],