add web WASM Playwright harness for UI driving

tools/ui/ — scripts + Playwright config that let Claude Code (and
humans) drive the Flutter WASM build in a real browser. The point is
to avoid screenshot round-trips: every interactive widget in clide
ships a Semantics wrapper anyway (a11y requirement), so the automation
layer just queries the `flt-semantics[aria-label]` DOM.

  * build.sh — `flutter build web --wasm` from app/
  * serve.sh — `python3 -m http.server 4280` in the background.
    Before binding, kills any stale listener on the port (orphans
    from earlier failed runs no longer accumulate).
  * stop.sh — port-based kill; escalates to SIGKILL after 300ms.
    The pidfile is now advisory — port ownership is the source of
    truth.
  * driver.ts — `ClideDriver` class with `byLabel`, `click`, `type`,
    `readText`, `screenshot`, `dumpSemanticsTree`, and
    `waitUntilReady` that auto-clicks the `flt-semantics-placeholder`
    so the semantic tree is populated before queries.
  * tests/smoke.spec.ts — first driver test; asserts welcome +
    disconnected labels surface in the Semantics DOM.

Co-Authored-By: Claude <noreply@anthropic.com>

tools/ui/playwright.config.ts

@@ -0,0 +1,26 @@
+import { defineConfig, devices } from '@playwright/test';
+
+export default defineConfig({
+  testDir: './tests',
+  // Single-worker: the web server is local and global; parallel tests
+  // would race on the shared browser state.
+  workers: 1,
+  reporter: 'line',
+  use: {
+    baseURL: process.env.CLIDE_UI_URL ?? 'http://localhost:4280',
+    trace: 'retain-on-failure',
+    screenshot: 'only-on-failure',
+    video: 'retain-on-failure',
+  },
+  projects: [
+    {
+      name: 'chromium',
+      use: {
+        ...devices['Desktop Chrome'],
+        // WASM + CanvasKit + semantics tree runs headless fine; no extra
+        // launch args needed for our driver surface.
+      },
+    },
+  ],
+  outputDir: 'out',
+});