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/pages/client/home/Home.tsx

@@ -27,8 +27,11 @@ import { useVirtualizer } from '@tanstack/react-virtual';
 import { useAtom, useAtomValue } from 'jotai';
 import { selectAtom } from 'jotai/utils';
 import FocusTrap from 'focus-trap-react';
-import { Unread } from '../../../../types/matrix/room';
-import { factoryRoomIdByActivity, factoryRoomIdByAtoZ } from '../../../utils/sort';
+import {
+  factoryRoomIdByActivity,
+  factoryRoomIdByAtoZ,
+  factoryRoomIdByUnread,
+} from '../../../utils/sort';
 import {
   NavButton,
   NavCategory,
@@ -210,17 +213,6 @@ function HomeEmpty() {
   );
 }
 
-const factoryRoomIdByUnread =
-  (roomToUnread: Map<string, Unread>) =>
-  (aId: string, bId: string): number => {
-    const aUnread = roomToUnread.get(aId);
-    const bUnread = roomToUnread.get(bId);
-    const aHas = (aUnread?.total ?? 0) > 0;
-    const bHas = (bUnread?.total ?? 0) > 0;
-    if (aHas !== bHas) return aHas ? -1 : 1;
-    return (bUnread?.total ?? 0) - (aUnread?.total ?? 0);
-  };
-
 const DEFAULT_CATEGORY_ID = makeNavCategoryId('home', 'room');
 const FAVORITES_CATEGORY_ID = makeNavCategoryId('home', 'favorite');
 const LOW_PRIORITY_CATEGORY_ID = makeNavCategoryId('home', 'lowpriority');
@@ -331,7 +323,7 @@ export function Home() {
     } else if (homeRoomSort === 'alpha') {
       comparator = factoryRoomIdByAtoZ(mx);
     } else if (homeRoomSort === 'unread') {
-      comparator = factoryRoomIdByUnread(roomToUnread);
+      comparator = factoryRoomIdByUnread(roomToUnread, mx);
     } else {
       comparator = factoryRoomIdByActivity(mx);
     }