feat(web): add useDelayed rune helper

Boolean mirror that flips true only after the source stays true for
delayMs. Used to hide the skeleton on sub-100ms cache hits so
back-navigation feels instant.

web/vitest.config.ts

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