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/ChatView.swift

@@ -1,27 +1,61 @@
 import SwiftUI
 import VoiceCatCore
 
+/// One row of the combined chat/activity timeline. Mirrors the macOS/Windows clients, which
+/// collapse chat messages and activity events into a single scrolling log (chat in normal
+/// text, activity events in gray).
+private enum TimelineItem: Identifiable {
+    case message(ChatMessage)
+    case activity(ActivityEntry)
+
+    var id: UUID {
+        switch self {
+        case .message(let m): return m.id
+        case .activity(let a): return a.id
+        }
+    }
+
+    var timestamp: Date {
+        switch self {
+        case .message(let m): return m.timestamp
+        case .activity(let a): return a.timestamp
+        }
+    }
+}
+
 struct ChatView: View {
     @Bindable var session: SessionState
     @State private var composeText = ""
     @State private var scope: VoiceCatTextScope = .channel
     @State private var privateTargetId: UInt32 = 0
 
+    private var timeline: [TimelineItem] {
+        let merged = session.messages.map(TimelineItem.message)
+            + session.activityLog.map(TimelineItem.activity)
+        return merged.sorted { $0.timestamp < $1.timestamp }
+    }
+
     var body: some View {
         VStack(spacing: 0) {
-            // Message list
+            // Combined chat + activity timeline
             ScrollViewReader { proxy in
                 ScrollView {
                     LazyVStack(alignment: .leading, spacing: 8) {
-                        ForEach(session.messages) { msg in
-                            ChatBubble(message: msg)
-                                .id(msg.id)
+                        ForEach(timeline) { item in
+                            switch item {
+                            case .message(let msg):
+                                ChatBubble(message: msg)
+                                    .id(item.id)
+                            case .activity(let entry):
+                                ActivityRow(entry: entry)
+                                    .id(item.id)
+                            }
                         }
                     }
                     .padding()
                 }
-                .onChange(of: session.messages.count) { _, _ in
-                    if let last = session.messages.last {
+                .onChange(of: session.messages.count + session.activityLog.count) { _, _ in
+                    if let last = timeline.last {
                         proxy.scrollTo(last.id, anchor: .bottom)
                     }
                 }
@@ -92,3 +126,34 @@ private struct ChatBubble: View {
         .accessibilityLabel("\(message.senderName) at \(timeString): \(message.text)")
     }
 }
+
+/// A compact, gray activity row interleaved into the chat timeline (joins/leaves, talk state,
+/// streams, server mute, etc.). Matches the "activity = gray" convention of the macOS/Windows
+/// unified logs.
+private struct ActivityRow: View {
+    let entry: ActivityEntry
+
+    private static let timeFormatter: DateFormatter = {
+        let fmt = DateFormatter()
+        fmt.dateStyle = .none
+        fmt.timeStyle = .short
+        return fmt
+    }()
+
+    private var timeString: String { Self.timeFormatter.string(from: entry.timestamp) }
+
+    var body: some View {
+        HStack(alignment: .top, spacing: 6) {
+            Text(timeString)
+                .font(.caption2)
+                .foregroundStyle(.tertiary)
+                .monospacedDigit()
+            Text(entry.text)
+                .font(.caption)
+                .foregroundStyle(.secondary)
+        }
+        .frame(maxWidth: .infinity, alignment: .leading)
+        .accessibilityElement(children: .combine)
+        .accessibilityLabel("\(timeString): \(entry.text)")
+    }
+}