DP16: add typed get/setAccountData helpers, remove ~19 any-casts

Add centralized typed helpers in src/app/utils/accountData.ts:
- getAccountData<T>(mx, eventType): T | undefined (returns content)
- setAccountData<T>(mx, eventType, content): Promise<void>

These wrap the single `as any` cast needed because matrix-js-sdk's typed
overloads reject the fork's custom account-data event names. Every call site
now stays fully typed on its content shape.

Route all account-data reads/writes that previously used
`(mx as any).getAccountData/setAccountData` or `mx.getAccountData(... as any)`
through the helpers (or, where a MatrixEvent is needed, through the existing
utils/room.ts getAccountData whose param is widened to accept string keys).
No behavior change: same event types, same content shapes.

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

src/app/features/lobby/Lobby.tsx

@@ -41,6 +41,7 @@ import { StateEvent } from '../../../types/matrix/room';
 import { CanDropCallback, useDnDMonitor } from './DnD';
 import { ASCIILexicalTable, orderKeys } from '../../utils/ASCIILexicalTable';
 import { getStateEvent } from '../../utils/room';
+import { setAccountData } from '../../utils/accountData';
 import { useClosedLobbyCategoriesAtom } from '../../state/hooks/closedLobbyCategories';
 import {
   makeCinnySpacesContent,
@@ -427,7 +428,7 @@ export function Lobby() {
         newItems.push(rId);
       }
       const newSpacesContent = makeCinnySpacesContent(mx, newItems);
-      mx.setAccountData(AccountDataEvent.CinnySpaces as any, newSpacesContent as any);
+      setAccountData(mx, AccountDataEvent.CinnySpaces, newSpacesContent);
     },
     [mx, sidebarItems, sidebarSpaces],
   );