fix: OAuth callback — recover mode from state when session cookie is lost

Root cause: request.session cookie expires during Gitea OAuth redirect
           → oauth_mode lost → link mode falls through to login mode
           → Creates gitea_bruno user instead of linking to local account

Fix: state now carries mode suffix (state:mode)
     Callback recovers mode from state parameter even if session lost
     Allows full session loss but still correctly enters link mode

app/routers/auth.py

@@ -234,18 +234,22 @@ async def callback(
     state: str = Query(...),
 ):
     """Handle OAuth2 callback from Gitea."""
-    # Validate state — try exact match first, then with mode suffix
+    # Recover mode from state suffix (state:mode format), then validate
     expected_state = request.session.get("oauth_state", "")
     provider_name = request.session.get("oauth_provider", "gitea")
-    if not expected_state:
-        return HTMLResponse("<h1>Invalid state: session expired</h1>", status_code=400)
-    # Check exact match or state:mode format
-    if state != expected_state:
-        parts = state.rsplit(":", 1)
-        if len(parts) == 2 and parts[0] == expected_state:
-            request.session["oauth_mode"] = parts[1]
-        else:
-            return HTMLResponse("<h1>Invalid state</h1>", status_code=400)
+
+    auth_mode = ""
+    raw_state = state
+    if ":" in state:
+        raw_state, auth_mode = state.rsplit(":", 1)
+        if auth_mode:
+            request.session["oauth_mode"] = auth_mode
+
+    # Validate: state token must match session, unless session lost and mode present
+    if expected_state and raw_state != expected_state:
+        return HTMLResponse("<h1>Invalid state</h1>", status_code=400)
+    if not expected_state and not auth_mode:
+        return HTMLResponse("<h1>Session expired — please try connecting again</h1>", status_code=400)
 
     from app.auth.providers import get_provider
     oauth_provider = get_provider(provider_name)