build(web): add TanStack Query + Testing Library deps

Wires @tanstack/svelte-query for the data layer and
@testing-library/{svelte,jest-dom} for component-level tests. Registers
the jest-dom matchers via a new vitest setup file.

web/vitest.config.ts

@@ -5,6 +5,7 @@ export default defineConfig({
   plugins: [sveltekit()],
   test: {
     environment: 'jsdom',
-    include: ['src/**/*.test.ts']
+    include: ['src/**/*.test.ts'],
+    setupFiles: ['./vitest.setup.ts']
   }
 });