Show the OIDC profile picture with an identicon fallback

GET /api/me now carries the profile picture URL when the provider
supplies an absolute http(s) one; the SPA shows it in the lobby navbar
and draws a deterministic identicon from the sub otherwise, falling back
to the identicon when the image fails to load.

The identicon is a 5x5 mirrored grid seeded by an inline FNV-1a hash so
it stays stable across Rust versions and is unit-testable on the host.
The dev mock IdP hands alice and bob a picture claim.

server/packages/tavolo-platform/tests/test_routes.py

@@ -310,7 +310,10 @@ class MeRouteTest(unittest.TestCase):
             with oidc_user(platform.oidc, "alice"):
                 response = await client.get("/api/me")
             self.assertEqual(200, response.status_code)
-            self.assertEqual({"sub": "alice", "name": "alice"}, response.json())
+            self.assertEqual(
+                {"sub": "alice", "name": "alice", "picture": None},
+                response.json(),
+            )
 
     @async_test
     async def test_me_unauthenticated(self) -> None: