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/utils/room.ts

@@ -45,7 +45,7 @@ export const getStateEvents = (room: Room, eventType: StateEvent): MatrixEvent[]
 
 export const getAccountData = (
   mx: MatrixClient,
-  eventType: AccountDataEvent,
+  eventType: AccountDataEvent | string,
 ): MatrixEvent | undefined => mx.getAccountData(eventType as any);
 
 export const getMDirects = (mDirectEvent: MatrixEvent): Set<string> => {