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/plugins/recent-emoji.ts

@@ -1,5 +1,6 @@
 import { MatrixClient } from 'matrix-js-sdk';
 import { getAccountData } from '../utils/room';
+import { setAccountData } from '../utils/accountData';
 import { IEmoji, emojis } from './emoji';
 import { AccountDataEvent } from '../../types/matrix/accountData';
 
@@ -39,7 +40,7 @@ export function addRecentEmoji(mx: MatrixClient, unicode: string) {
     entry[1] += 1;
   }
   recentEmoji.unshift(entry);
-  (mx as any).setAccountData(AccountDataEvent.ElementRecentEmoji, {
+  setAccountData(mx, AccountDataEvent.ElementRecentEmoji, {
     recent_emoji: recentEmoji.slice(0, 100),
   });
 }