Prefer preferred_username for the displayed player name

The free-form "name" claim can change and two accounts can share it;
preferred_username is the account's stable handle. display_name() now
falls back preferred_username -> name -> email -> sub, with tests
pinning the claim priority.

server/packages/tavolo-platform/src/tavolo/platform/auth.py

@@ -47,8 +47,13 @@ def get_ws_user(ws: WebSocket) -> Optional[OIDCUser]:
 
 
 def display_name(user: OIDCUser) -> str:
-    """Best-effort human-readable name for a user."""
-    for key in ("name", "preferred_username", "email"):
+    """Best-effort human-readable name for a user.
+
+    ``preferred_username`` is preferred: it is the account's stable handle,
+    while ``name`` is a free-form display name a user may change (and two
+    accounts can share). ``email`` is a last resort before the ``sub``.
+    """
+    for key in ("preferred_username", "name", "email"):
         value = user.get(key)
         if isinstance(value, str) and value:
             return value

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

@@ -11,11 +11,17 @@ import unittest
 from types import SimpleNamespace
 from typing import Any
 
-from kaya.oidc import OIDCConfig, OIDCProviders
+from kaya.oidc import OIDCConfig, OIDCProviders, OIDCUser
 from kaya.session import InMemorySessionStore, Session, SessionMixin
 
 from helpers import async_test
-from tavolo.platform.auth import current_user, get_ws_user, require_auth, session_user
+from tavolo.platform.auth import (
+    current_user,
+    display_name,
+    get_ws_user,
+    require_auth,
+    session_user,
+)
 
 
 def _providers(*names: str) -> OIDCProviders:
@@ -87,6 +93,32 @@ class GetWsUserTest(unittest.TestCase):
         self.assertIsNone(get_ws_user(SimpleNamespace()))
 
 
+class DisplayNameTest(unittest.TestCase):
+    """The claim priority used for the name shown at the table."""
+
+    def test_prefers_preferred_username_over_name(self) -> None:
+        user = OIDCUser(
+            {"sub": "alice", "preferred_username": "alice", "name": "Alice"}
+        )
+        self.assertEqual("alice", display_name(user))
+
+    def test_falls_back_to_name(self) -> None:
+        user = OIDCUser({"sub": "alice", "name": "Alice"})
+        self.assertEqual("Alice", display_name(user))
+
+    def test_falls_back_to_email(self) -> None:
+        user = OIDCUser({"sub": "alice", "email": "alice@example.com"})
+        self.assertEqual("alice@example.com", display_name(user))
+
+    def test_falls_back_to_sub(self) -> None:
+        user = OIDCUser({"sub": "alice"})
+        self.assertEqual("alice", display_name(user))
+
+    def test_ignores_empty_and_non_string_claims(self) -> None:
+        user = OIDCUser({"sub": "alice", "preferred_username": "", "name": 42})
+        self.assertEqual("alice", display_name(user))
+
+
 class CurrentUserTest(unittest.TestCase):
     def test_anonymous_session_has_no_user(self) -> None:
         self.assertIsNone(current_user(_providers("default"), _ctx({})))