fix(ux): cap the in-app toast stack; stable "Unread First" room sort

- Toast queue: a burst of notifications appended unboundedly and could cover the
  viewport. Cap at 5 in the atom writer, dropping the OLDEST non-sticky toast
  (sticky = action toasts requiring a click, never dropped). The drop scan
  excludes the just-appended newest (`length - 1` bound) so a fresh toast is
  never the one eaten when the cap is full of stickies — it stretches instead.
  Container gains a maxHeight + overflowY safety net and scrolls the newest
  (bottom) toast into view if the stack ever overflows. +4 unit tests incl. the
  cap-full-of-stickies boundary.

- "Unread First" room sort left the entire read tail (all counts tie at 0) in
  arbitrary Map order. factoryRoomIdByUnread now breaks ties by recent activity.
  Relocated from Home.tsx (module-private) to utils/sort.ts (exported, pure) and
  unit-tested (equal-count and read-tail cases fall back to activity).

Bug-hunt findings from LOTUS_TODO. Three review passes: the second caught that
the cap could silently drop the newest notification when full of stickies (real
bug, untested boundary) — fixed and covered; a third traced the corrected loop.
Gate-green (tsc, eslint, prettier, 920 tests, build).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/utils/sort.ts

@@ -1,4 +1,5 @@
 import { MatrixClient } from 'matrix-js-sdk';
+import { Unread } from '../../types/matrix/room';
 
 export type SortFunc<T> = (a: T, b: T) => number;
 
@@ -42,6 +43,26 @@ export const factoryRoomIdByUnreadCount =
     return bT - aT;
   };
 
+// "Unread First": rooms with unread sort before those without, then by unread
+// count desc, then — crucially for the large all-read tail where counts tie —
+// by recent activity, so it isn't left in arbitrary order.
+export const factoryRoomIdByUnread = (
+  roomToUnread: Map<string, Unread>,
+  mx: MatrixClient,
+): SortFunc<string> => {
+  const byActivity = factoryRoomIdByActivity(mx);
+  return (a, b) => {
+    const aUnread = roomToUnread.get(a);
+    const bUnread = roomToUnread.get(b);
+    const aHas = (aUnread?.total ?? 0) > 0;
+    const bHas = (bUnread?.total ?? 0) > 0;
+    if (aHas !== bHas) return aHas ? -1 : 1;
+    const byCount = (bUnread?.total ?? 0) - (aUnread?.total ?? 0);
+    if (byCount !== 0) return byCount;
+    return byActivity(a, b);
+  };
+};
+
 export const byTsOldToNew: SortFunc<number> = (a, b) => a - b;
 
 export const byOrderKey: SortFunc<string | undefined> = (a, b) => {