feat(rooms): Mark as Unread (MSC2867) + Low Priority rooms

Two Matrix protocol gaps (Phase A), gate-green (683 tests):
- Mark as Unread: m.marked_unread room account data (+ com.famedly.marked_unread
  fallback), a new markedUnreadAtom binder that seeds from account data and
  clears on our own read receipt (MSC2867). RoomNavItem gains Mark as Unread /
  Read menu items and lights the row dot for a marked room. Tested.
- Low Priority: m.lowpriority room tag mirroring favourites — a context-menu
  toggle (mutually exclusive with Favorite) and a collapsed Low Priority
  category sorted to the bottom of the Home room list.

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

src/app/features/room-nav/RoomNavItem.tsx

@@ -38,6 +38,7 @@ import { nameInitials } from '../../utils/common';
 import { useMatrixClient } from '../../hooks/useMatrixClient';
 import { useRoomUnread } from '../../state/hooks/unread';
 import { roomToUnreadAtom } from '../../state/room/roomToUnread';
+import { markedUnreadAtom, setMarkedUnread } from '../../state/room/markedUnread';
 import { getPowersLevelFromMatrixEvent, usePowerLevels } from '../../hooks/usePowerLevels';
 import { markAsRead } from '../../utils/notifications';
 import { UseStateProvider } from '../../components/UseStateProvider';
@@ -329,18 +330,39 @@ const RoomNavItemMenu = forwardRef<HTMLDivElement, RoomNavItemMenuProps>(
     const isServerNotice = room.getType() === 'm.server_notice';
 
     const isFavorite = !!room.tags?.['m.favourite'];
+    const isLowPriority = !!room.tags?.['m.lowpriority'];
 
     const handleToggleFavorite = () => {
       if (isFavorite) {
         mx.deleteRoomTag(room.roomId, 'm.favourite');
       } else {
+        // Favourite and low-priority are mutually exclusive.
+        if (isLowPriority) mx.deleteRoomTag(room.roomId, 'm.lowpriority');
         mx.setRoomTag(room.roomId, 'm.favourite', { order: 0.5 });
       }
       requestClose();
     };
 
+    const handleToggleLowPriority = () => {
+      if (isLowPriority) {
+        mx.deleteRoomTag(room.roomId, 'm.lowpriority');
+      } else {
+        if (isFavorite) mx.deleteRoomTag(room.roomId, 'm.favourite');
+        mx.setRoomTag(room.roomId, 'm.lowpriority', { order: 0.5 });
+      }
+      requestClose();
+    };
+
+    const markedUnread = useAtomValue(markedUnreadAtom).has(room.roomId);
+
     const handleMarkAsRead = () => {
       markAsRead(mx, room.roomId, hideActivity);
+      if (markedUnread) setMarkedUnread(mx, room.roomId, false).catch(() => undefined);
+      requestClose();
+    };
+
+    const handleMarkAsUnread = () => {
+      setMarkedUnread(mx, room.roomId, true).catch(() => undefined);
       requestClose();
     };
 
@@ -393,12 +415,23 @@ const RoomNavItemMenu = forwardRef<HTMLDivElement, RoomNavItemMenuProps>(
             size="300"
             after={<Icon size="100" src={Icons.CheckTwice} />}
             radii="300"
-            disabled={!unread}
+            disabled={!unread && !markedUnread}
           >
             <Text style={{ flexGrow: 1 }} as="span" size="T300" truncate>
               Mark as Read
             </Text>
           </MenuItem>
+          <MenuItem
+            onClick={handleMarkAsUnread}
+            size="300"
+            after={<Icon size="100" src={Icons.MessageUnread} />}
+            radii="300"
+            disabled={!!unread || markedUnread}
+          >
+            <Text style={{ flexGrow: 1 }} as="span" size="T300" truncate>
+              Mark as Unread
+            </Text>
+          </MenuItem>
           <RoomNotificationModeSwitcher roomId={room.roomId} value={notificationMode}>
             {(handleOpen, opened, changing) => (
               <MenuItem
@@ -493,6 +526,17 @@ const RoomNavItemMenu = forwardRef<HTMLDivElement, RoomNavItemMenuProps>(
               {isFavorite ? 'Remove from Favorites' : 'Add to Favorites'}
             </Text>
           </MenuItem>
+          <MenuItem
+            onClick={handleToggleLowPriority}
+            size="300"
+            after={<Icon size="100" src={Icons.ChevronBottom} />}
+            radii="300"
+            aria-pressed={isLowPriority}
+          >
+            <Text style={{ flexGrow: 1 }} as="span" size="T300" truncate>
+              {isLowPriority ? 'Remove from Low Priority' : 'Add to Low Priority'}
+            </Text>
+          </MenuItem>
           <MenuItem
             onClick={handleInvite}
             variant="Primary"
@@ -610,6 +654,10 @@ function RoomNavItem_({
   const [menuAnchor, setMenuAnchor] = useState<RectCords>();
   const [renameDialog, setRenameDialog] = useState(false);
   const unread = useRoomUnread(room.roomId, roomToUnreadAtom);
+  // MSC2867: an explicit "mark as unread" lights the row even with no unread
+  // count. `hasUnread` drives the bold name / icon emphasis below.
+  const markedUnread = useAtomValue(markedUnreadAtom).has(room.roomId);
+  const hasUnread = !!unread || markedUnread;
   const typingMember = useRoomTypingMember(room.roomId).filter(
     (receipt) => receipt.userId !== mx.getUserId(),
   );
@@ -692,7 +740,7 @@ function RoomNavItem_({
     <NavItem
       variant="Background"
       radii="400"
-      highlight={unread !== undefined}
+      highlight={hasUnread}
       aria-selected={selected}
       data-hover={!!menuAnchor}
       onContextMenu={handleContextMenu}
@@ -721,7 +769,7 @@ function RoomNavItem_({
               ) : (
                 <RoomIcon
                   style={{
-                    opacity: unread ? config.opacity.P500 : config.opacity.P300,
+                    opacity: hasUnread ? config.opacity.P500 : config.opacity.P300,
                   }}
                   filled={selected}
                   size="100"
@@ -732,7 +780,7 @@ function RoomNavItem_({
             </Avatar>
             <Box as="span" direction="Column" grow="Yes" style={{ minWidth: 0 }}>
               <Box as="span" grow="Yes" alignItems="Center" gap="100">
-                <Text priority={unread ? '500' : '300'} as="span" size="Inherit" truncate>
+                <Text priority={hasUnread ? '500' : '300'} as="span" size="Inherit" truncate>
                   {roomName}
                 </Text>
                 {hasLocalName && (
@@ -773,7 +821,7 @@ function RoomNavItem_({
                 </Box>
               )}
             </Box>
-            {!optionsVisible && !unread && !selected && typingMember.length > 0 && (
+            {!optionsVisible && !hasUnread && !selected && typingMember.length > 0 && (
               <Badge size="300" variant="Secondary" fill="Soft" radii="Pill" outlined>
                 <TypingIndicator size="300" disableAnimation />
               </Badge>
@@ -783,6 +831,11 @@ function RoomNavItem_({
                 <UnreadBadge highlight={unread.highlight > 0} count={unread.total} />
               </UnreadBadgeCenter>
             )}
+            {!optionsVisible && !unread && markedUnread && (
+              <UnreadBadgeCenter>
+                <UnreadBadge highlight={false} count={0} />
+              </UnreadBadgeCenter>
+            )}
             {!optionsVisible && notificationMode !== RoomNotificationMode.Unset && (
               <Icon
                 size="50"