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/useThreadNotifications.ts

@@ -12,6 +12,7 @@ import {
 } from '../utils/threadNotifications';
 import { useMatrixClient } from './useMatrixClient';
 import { AsyncState, useAsyncCallback } from './useAsyncCallback';
+import { getAccountData, setAccountData } from '../utils/accountData';
 
 /** Read the current notification mode for a thread from the bound atom. */
 export function useThreadNotificationMode(
@@ -23,9 +24,7 @@ export function useThreadNotificationMode(
 }
 
 const readContent = (mx: MatrixClient): ThreadNotificationsContent =>
-  ((mx as any).getAccountData(AccountDataEvent.LotusThreadNotifications)?.getContent() as
-    | ThreadNotificationsContent
-    | undefined) ?? {};
+  getAccountData<ThreadNotificationsContent>(mx, AccountDataEvent.LotusThreadNotifications) ?? {};
 
 const getJoinedRoomIds = (mx: MatrixClient): Set<string> => {
   const joined = new Set<string>();
@@ -74,7 +73,7 @@ const writeThreadNotificationMode = async (
   // ALWAYS prune before persisting to keep account data bounded.
   const finalContent = pruneThreadNotifications(next, getJoinedRoomIds(mx), now);
 
-  await (mx as any).setAccountData(AccountDataEvent.LotusThreadNotifications, finalContent);
+  await setAccountData(mx, AccountDataEvent.LotusThreadNotifications, finalContent);
 };
 
 export function useSetThreadNotificationMode(