Improve notification view filtering UX

- Add subtitle below the toolbar title to indicate the state of the filter
- Add settings icon to take user to the notification settings page, and
  thus make that more discoverable

Testing
-------

PASS

Device: iPhone 13 mini
iOS: 17.6.1
Coverage:
1. Switching back and forth between the notifications tab and other tabs
   causes subtitle to show/hide as expected in both filter options
   (all, friends)
2. Subtitle follows the friends filter
3. Subtitle shows after restarting the app
4. Settings icon appears and takes user to the notification setting view
5. Notification settings can be updated from that view.

Changelog-Changed: Improve notification view filtering UX
Closes: https://github.com/damus-io/damus/issues/2480
Signed-off-by: Daniel D’Aquino <daniel@daquino.me>

damus/ContentView.swift

@@ -77,7 +77,12 @@ struct ContentView: View {
     
     @State var active_sheet: Sheets? = nil
     @State var damus_state: DamusState!
-    @SceneStorage("ContentView.selected_timeline") var selected_timeline: Timeline = .home
+    @State var menu_subtitle: String? = nil
+    @SceneStorage("ContentView.selected_timeline") var selected_timeline: Timeline = .home {
+        willSet {
+            self.menu_subtitle = nil
+        }
+    }
     @State var muting: MuteItem? = nil
     @State var confirm_mute: Bool = false
     @State var hide_bar: Bool = false
@@ -159,9 +164,16 @@ struct ContentView: View {
         isSideBarOpened = false
     }
     
-    var timelineNavItem: Text {
-        return Text(timeline_name(selected_timeline))
-            .bold()
+    var timelineNavItem: some View {
+        VStack {
+            Text(timeline_name(selected_timeline))
+                .bold()
+            if let menu_subtitle {
+                Text(menu_subtitle)
+                    .font(.caption)
+                    .foregroundStyle(.secondary)
+            }
+        }
     }
     
     func MainContent(damus: DamusState) -> some View {
@@ -180,7 +192,7 @@ struct ContentView: View {
                 PostingTimelineView
                 
             case .notifications:
-                NotificationsView(state: damus, notifications: home.notifications)
+                NotificationsView(state: damus, notifications: home.notifications, subtitle: $menu_subtitle)
                 
             case .dms:
                 DirectMessagesView(damus_state: damus_state!, model: damus_state!.dms, settings: damus_state!.settings)