fix(invites): decline invites robustly (no 500, no ghost, friendly error)

Declining a remote invite could show a raw 'MatrixError: [500] Internal server
error' and appear to do nothing. Root causes were client-side: decline called
mx.leave unconditionally, so re-clicking after a slow federated leave hit an
already-left remote room that Synapse 500s on; the room was never forgotten so a
'leave' ghost lingered and re-invited a click; and the raw error string was shown.

Add a shared declineInvite(mx, roomId) helper that only leaves when still in the
room (invite/join/knock) and then forgets it (best-effort, first use of forget in
the app). Route the InviteCard decline and both 'Decline All' paths through it,
and replace the raw error with a friendly message (real error kept in console).

Tests: declineInvite covered (6 cases); typecheck + full suite + build clean.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/pages/client/inbox/Invites.tsx

@@ -47,6 +47,7 @@ import { nameInitials } from '../../../utils/common';
 import { RoomAvatar } from '../../../components/room-avatar';
 import {
   addRoomIdToMDirect,
+  declineInvite,
   getMxIdLocalPart,
   guessDmRoomUserId,
   rateLimitedActions,
@@ -179,8 +180,16 @@ function InviteCard({
       onNavigate(invite.roomId, invite.isSpace);
     }, [mx, invite, userId, onNavigate]),
   );
-  const [leaveState, leave] = useAsyncCallback<Record<string, never>, MatrixError, []>(
-    useCallback(() => mx.leave(invite.roomId), [mx, invite]),
+  const [leaveState, leave] = useAsyncCallback<void, Error, []>(
+    useCallback(async () => {
+      try {
+        await declineInvite(mx, invite.roomId);
+      } catch (e) {
+        // Surface a friendly message; keep the real error in the console.
+        console.warn('Failed to decline invite', invite.roomId, e);
+        throw e;
+      }
+    }, [mx, invite.roomId]),
   );
 
   const joining =
@@ -276,7 +285,7 @@ function InviteCard({
             )}
             {leaveState.status === AsyncStatus.Error && (
               <Text size="T200" style={{ color: color.Critical.Main }}>
-                {leaveState.error.message}
+                Couldn’t decline this invite — please try again.
               </Text>
             )}
           </Box>
@@ -481,7 +490,7 @@ function UnknownInvites({
     useCallback(async () => {
       const roomIds = invites.map((invite) => invite.roomId);
 
-      await rateLimitedActions(roomIds, (roomId) => mx.leave(roomId));
+      await rateLimitedActions(roomIds, (roomId) => declineInvite(mx, roomId));
     }, [mx, invites]),
   );
 
@@ -559,7 +568,7 @@ function SpamInvites({
     useCallback(async () => {
       const roomIds = invites.map((invite) => invite.roomId);
 
-      await rateLimitedActions(roomIds, (roomId) => mx.leave(roomId));
+      await rateLimitedActions(roomIds, (roomId) => declineInvite(mx, roomId));
     }, [mx, invites]),
   );