Also email room subscribers on the first unread message, not just mentions (#67)

The previous pass only emailed on a mention. Corrected scope: the
room's first unread message triggers one debounced email (same shape
as #66), and every mention additionally emails regardless of that
debounce, since a mention shouldn't get silently absorbed by an
earlier plain message's already-sent notification.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/components/RoomInfoPanel.tsx

@@ -398,8 +398,8 @@ export function RoomInfoPanel({
         <div className="room-info-section">
           <div className="toggle-row">
             <div className="toggle-label">
-              <span className="t">Email me on mentions</span>
-              <span className="d">Sent only while you're offline</span>
+              <span className="t">Email notifications</span>
+              <span className="d">New messages and mentions, while you're offline</span>
             </div>
             <label className="switch">
               <input