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/hooks/useAccountData.ts

@@ -2,11 +2,12 @@ import { MatrixEvent } from 'matrix-js-sdk';
 import { useState, useCallback } from 'react';
 import { useMatrixClient } from './useMatrixClient';
 import { useAccountDataCallback } from './useAccountDataCallback';
+import { getAccountData } from '../utils/room';
 
 export function useAccountData(eventType: string): MatrixEvent | undefined {
   const mx = useMatrixClient();
-  const [event, setEvent] = useState<MatrixEvent | undefined>(
-    () => (mx as any).getAccountData(eventType) as MatrixEvent | undefined,
+  const [event, setEvent] = useState<MatrixEvent | undefined>(() =>
+    getAccountData(mx, eventType),
   );
 
   useAccountDataCallback(