feat(ios): iPhone channel drill-down, fix chat compose box, unify chat+activity

- Channels tab is now a drill-down on iPhone: ChannelBrowserView lists top-level
  channels; ChannelDetailView shows the people in a channel, its sub-channels, and
  an explicit Join button (with password prompt). iPad split view unchanged.
- Extract self-contained UserRow (context menu + sheets) from UserListView so admin
  actions are reused in the drill-down.
- Fix off-screen chat compose box: pin VoiceControlsView via per-tab
  .safeAreaInset(edge: .bottom) instead of a floating overlay, so it reserves layout
  space above the tab bar (keeping the compose box visible, cooperating with keyboard
  avoidance) without covering the tab bar buttons.
- Collapse Activity into Chat like macOS/Windows: ChatView renders a merged,
  time-sorted timeline of messages + activity (activity rows in gray); remove the
  Activity tab and ActivityLogView.
- Label the RPSystemBroadcastPickerView inner UIButton for VoiceOver
  ("Share/Stop sharing screen audio") instead of relying on an outer SwiftUI label.

clients/apple/iOS/VoiceCatiOS/Views/MainView.swift

@@ -24,26 +24,33 @@ private struct iPhoneMainView: View {
 
     var body: some View {
         TabView {
-            ChannelTreeView(session: session)
+            ChannelBrowserView(session: session)
+                .voiceControlsBar(session)
                 .tabItem {
                     Label("Channels", systemImage: "list.bullet.indent")
                 }
             ChatView(session: session)
+                .voiceControlsBar(session)
                 .tabItem {
                     Label("Chat", systemImage: "message")
                 }
-            ActivityLogView(session: session)
-                .tabItem {
-                    Label("Activity", systemImage: "bell")
-                }
             SettingsView(session: session)
+                .voiceControlsBar(session)
                 .tabItem {
                     Label("Settings", systemImage: "gear")
                 }
         }
-        .overlay(alignment: .bottom) {
+    }
+}
+
+private extension View {
+    /// Pin the shared voice controls just above the tab bar, *inside each tab's content area*.
+    /// Applying this per-tab (rather than to the TabView itself) reserves layout space above
+    /// the tab bar — keeping ChatView's compose box visible and cooperating with keyboard
+    /// avoidance — without the bar covering the tab bar's buttons.
+    func voiceControlsBar(_ session: SessionState) -> some View {
+        safeAreaInset(edge: .bottom) {
             VoiceControlsView(session: session)
-                .padding(.bottom, 56) // above tab bar
         }
     }
 }