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/ChannelBrowserView.swift

@@ -0,0 +1,91 @@
+import SwiftUI
+import VoiceCatCore
+
+/// iPhone channels tab: a drill-down browser. The root list shows only top-level channels;
+/// tapping (or VoiceOver-activating) a channel pushes `ChannelDetailView`, which shows the
+/// people in it and any sub-channels. Joining is an explicit action inside the detail view.
+struct ChannelBrowserView: View {
+    @Bindable var session: SessionState
+    @State private var showCreateChannel = false
+
+    var body: some View {
+        NavigationStack {
+            List {
+                ForEach(rootChannels) { ch in
+                    NavigationLink {
+                        ChannelDetailView(channel: ch, session: session)
+                    } label: {
+                        ChannelRow(channel: ch, session: session)
+                    }
+                    .swipeActions(edge: .trailing) {
+                        if session.permissions.isAdmin {
+                            Button(role: .destructive) {
+                                session.deleteChannel(ch.id)
+                            } label: {
+                                Label("Delete", systemImage: "trash")
+                            }
+                        }
+                    }
+                }
+            }
+            .navigationTitle("Channels")
+            .toolbar {
+                if session.permissions.canCreateTempChannel || session.permissions.isAdmin {
+                    ToolbarItem(placement: .primaryAction) {
+                        Button {
+                            showCreateChannel = true
+                        } label: {
+                            Image(systemName: "plus")
+                        }
+                        .accessibilityLabel("Create channel")
+                    }
+                }
+            }
+            .sheet(isPresented: $showCreateChannel) {
+                ChannelEditView(channelId: nil, session: session)
+            }
+        }
+    }
+
+    private var rootChannels: [Channel] {
+        session.channels
+            .filter { $0.parentId == 0 }
+            .sorted { $0.name < $1.name }
+    }
+}
+
+/// Shared channel row used by the iPhone browser and detail views. Mirrors the look of the
+/// iPad `ChannelTreeView` row but keyed on a `Channel` rather than a tree `ChannelNode`.
+struct ChannelRow: View {
+    let channel: Channel
+    let session: SessionState
+
+    var body: some View {
+        let isCurrent = session.currentChannelId == channel.id
+        let usersHere = session.users.filter { $0.channelId == channel.id }
+
+        HStack(spacing: 8) {
+            Image(systemName: channel.passwordProtected ? "lock.fill" : "number")
+                .foregroundStyle(isCurrent ? .blue : .secondary)
+                .imageScale(.small)
+            VStack(alignment: .leading, spacing: 1) {
+                Text(channel.name)
+                    .fontWeight(isCurrent ? .semibold : .regular)
+                if !channel.topic.isEmpty {
+                    Text(channel.topic)
+                        .font(.caption2)
+                        .foregroundStyle(.secondary)
+                        .lineLimit(1)
+                }
+            }
+            Spacer()
+            if !usersHere.isEmpty {
+                Text("\(usersHere.count)")
+                    .font(.caption)
+                    .foregroundStyle(.secondary)
+            }
+        }
+        .accessibilityElement(children: .combine)
+        .accessibilityLabel("\(channel.name)\(isCurrent ? ", current" : "")\(channel.passwordProtected ? ", password protected" : "")\(!usersHere.isEmpty ? ", \(usersHere.count) users" : "")")
+    }
+}