Make the frontend an installable PWA

Add a web app manifest and an install-only service worker (no offline
caching: live state always comes from the server and hashed asset names
change every build), registered from the WASM bundle. The worker must
live at the site root for its scope to cover the app, and Granian only
serves /static and /assets, so tavolo/static.py answers
/manifest.webmanifest and /sw.js from STATIC_DIR with the content types
browsers require; index.html gains the manifest link, icons, theme-color
and Apple standalone meta tags.

Icons are committed under web/assets/icons/ with vector sources and a
make-icons.sh regeneration script, mirroring the card art workflow.

server/tests/test_routes_me.py

@@ -81,6 +81,46 @@ class StaticRouteTest(unittest.TestCase):
                     self.assertEqual(200, fallback.status_code)
                     self.assertIn(b"spa", fallback.content)
 
+    @async_test
+    async def test_serves_pwa_manifest_and_worker(self) -> None:
+        """The manifest and the service worker are served from the site
+        root, with the content types browsers require, instead of falling
+        through to the app shell."""
+        with tempfile.TemporaryDirectory() as dist:
+            (Path(dist) / "index.html").write_text("<html>spa</html>")
+            (Path(dist) / "manifest.webmanifest").write_text('{"name": "Tavolo"}')
+            (Path(dist) / "sw.js").write_text("self.addEventListener('fetch', () => {});")
+
+            patched = dataclasses.replace(settings, static_dir=dist)
+            with mock.patch("tavolo.static.settings", patched):
+                transport = ASGITransport(app=app)
+                async with AsyncClient(transport=transport, base_url="http://127.0.0.1") as client:
+                    manifest = await client.get("/manifest.webmanifest")
+                    self.assertEqual(200, manifest.status_code)
+                    self.assertEqual("application/manifest+json", manifest.headers["content-type"])
+                    self.assertEqual('{"name": "Tavolo"}', manifest.text)
+
+                    worker = await client.get("/sw.js")
+                    self.assertEqual(200, worker.status_code)
+                    self.assertEqual("text/javascript; charset=utf-8", worker.headers["content-type"])
+                    # The worker script itself must never be cached stale.
+                    self.assertEqual("no-cache", worker.headers["cache-control"])
+                    self.assertIn(b"addEventListener", worker.content)
+
+    @async_test
+    async def test_missing_pwa_files_return_404(self) -> None:
+        """A build without the PWA files must 404 those exact paths rather
+        than answer them with the shell (the wrong content type)."""
+        with tempfile.TemporaryDirectory() as dist:
+            (Path(dist) / "index.html").write_text("<html>spa</html>")
+
+            patched = dataclasses.replace(settings, static_dir=dist)
+            with mock.patch("tavolo.static.settings", patched):
+                transport = ASGITransport(app=app)
+                async with AsyncClient(transport=transport, base_url="http://127.0.0.1") as client:
+                    self.assertEqual(404, (await client.get("/manifest.webmanifest")).status_code)
+                    self.assertEqual(404, (await client.get("/sw.js")).status_code)
+
     @async_test
     async def test_missing_dist_returns_404(self) -> None:
         patched = dataclasses.replace(settings, static_dir="/nonexistent-dist")