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/ChannelDetailView.swift

@@ -0,0 +1,88 @@
+import SwiftUI
+import VoiceCatCore
+
+/// The drilled-into view for a single channel: a Join control, the people currently in the
+/// channel, and any sub-channels (each drilling deeper via a nested `ChannelDetailView`).
+struct ChannelDetailView: View {
+    let channel: Channel
+    @Bindable var session: SessionState
+
+    @State private var showPasswordPrompt = false
+    @State private var password = ""
+
+    private var isCurrent: Bool { session.currentChannelId == channel.id }
+    private var people: [User] { session.users.filter { $0.channelId == channel.id } }
+    private var subchannels: [Channel] {
+        session.channels.filter { $0.parentId == channel.id }.sorted { $0.name < $1.name }
+    }
+
+    var body: some View {
+        List {
+            Section {
+                if isCurrent {
+                    Label("You're here", systemImage: "checkmark.circle.fill")
+                        .foregroundStyle(.green)
+                        .accessibilityLabel("You are in this channel")
+                } else {
+                    Button {
+                        join()
+                    } label: {
+                        Label("Join Channel", systemImage: "arrow.right.circle.fill")
+                    }
+                    .accessibilityLabel("Join \(channel.name)")
+                }
+            }
+
+            Section("People") {
+                if people.isEmpty {
+                    Text("No one here yet.")
+                        .foregroundStyle(.secondary)
+                } else {
+                    ForEach(people) { user in
+                        UserRow(user: user, session: session)
+                    }
+                }
+            }
+
+            if !subchannels.isEmpty {
+                Section("Channels") {
+                    ForEach(subchannels) { sub in
+                        NavigationLink {
+                            ChannelDetailView(channel: sub, session: session)
+                        } label: {
+                            ChannelRow(channel: sub, session: session)
+                        }
+                        .swipeActions(edge: .trailing) {
+                            if session.permissions.isAdmin {
+                                Button(role: .destructive) {
+                                    session.deleteChannel(sub.id)
+                                } label: {
+                                    Label("Delete", systemImage: "trash")
+                                }
+                            }
+                        }
+                    }
+                }
+            }
+        }
+        .navigationTitle(channel.name)
+        .navigationBarTitleDisplayMode(.inline)
+        .alert("Channel Password", isPresented: $showPasswordPrompt) {
+            SecureField("Password", text: $password)
+                .accessibilityLabel("Channel password")
+            Button("Join") {
+                session.joinChannel(channel.id, password: password)
+                password = ""
+            }
+            Button("Cancel", role: .cancel) { password = "" }
+        }
+    }
+
+    private func join() {
+        if channel.passwordProtected {
+            showPasswordPrompt = true
+        } else {
+            session.joinChannel(channel.id)
+        }
+    }
+}