Add Shared Media drawer and harden same-room navigation.

Widen the media panel, support skinny full-page mode, keep jump-to-latest and return-to-previous reliable, and stop same-room event jumps from remounting the outlet or yanking the sidebar.

src/app/state/mediaDrawer.ts

@@ -0,0 +1,17 @@
+import { atom } from 'jotai';
+
+/** Session flag for the Shared Media side drawer (mutually exclusive with Members). */
+export const isMediaDrawerAtom = atom(false);
+
+const scrollTops = new Map<string, number>();
+
+export const getMediaDrawerScrollTop = (roomId: string): number | undefined =>
+  scrollTops.get(roomId);
+
+export const setMediaDrawerScrollTop = (roomId: string, top: number): void => {
+  scrollTops.set(roomId, top);
+};
+
+export const clearMediaDrawerScrollTop = (roomId: string): void => {
+  scrollTops.delete(roomId);
+};

src/app/state/roomScrollCache.ts

@@ -30,3 +30,17 @@ export const saveRoomScrollState = (roomId: string, state: RoomScrollState): voi
 export const clearRoomScrollState = (roomId: string): void => {
   cache.delete(roomId);
 };
+
+/** Survives RoomTimeline remounts (e.g. clearing an event permalink URL). */
+const returnAnchors = new Map<string, string>();
+
+export const getRoomReturnAnchor = (roomId: string): string | undefined =>
+  returnAnchors.get(roomId);
+
+export const setRoomReturnAnchor = (roomId: string, eventId: string): void => {
+  returnAnchors.set(roomId, eventId);
+};
+
+export const clearRoomReturnAnchor = (roomId: string): void => {
+  returnAnchors.delete(roomId);
+};