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/UserRow.swift

@@ -0,0 +1,121 @@
+import SwiftUI
+import VoiceCatCore
+
+/// A single user row with its admin context menu and the sheets those actions present.
+/// Self-contained (owns its own sheet state) so it can be reused both by the iPad
+/// `UserListView` middle column and by the iPhone `ChannelDetailView` drill-down.
+struct UserRow: View {
+    let user: User
+    @Bindable var session: SessionState
+
+    @State private var activeSheet: ActiveSheet?
+
+    private enum ActiveSheet: Identifiable {
+        case tuning, ban, move, permissions
+        var id: Int { hashValue }
+    }
+
+    var body: some View {
+        UserRowView(user: user, isSelf: user.id == session.selfUserId)
+            .contextMenu { contextMenu }
+            .sheet(item: $activeSheet) { sheet in
+                switch sheet {
+                case .tuning:      PerUserTuningView(user: user, session: session)
+                case .ban:         BanUserView(user: user, session: session)
+                case .move:        MoveUserView(user: user, session: session)
+                case .permissions: PermissionsView(user: user, session: session)
+                }
+            }
+    }
+
+    @ViewBuilder
+    private var contextMenu: some View {
+        if user.id != session.selfUserId {
+            Button {
+                activeSheet = .tuning
+            } label: {
+                Label("Volume / NR", systemImage: "speaker.wave.2")
+            }
+            if session.permissions.canKick || session.permissions.isAdmin {
+                Divider()
+                Button {
+                    session.kickUser(user.id, reason: "")
+                } label: {
+                    Label("Kick", systemImage: "person.fill.xmark")
+                }
+                if session.permissions.canBan || session.permissions.isAdmin {
+                    Button(role: .destructive) {
+                        activeSheet = .ban
+                    } label: {
+                        Label("Ban…", systemImage: "nosign")
+                    }
+                }
+            }
+            if session.permissions.canMoveUsers || session.permissions.isAdmin {
+                Button {
+                    activeSheet = .move
+                } label: {
+                    Label("Move to channel…", systemImage: "arrow.right.circle")
+                }
+            }
+            if session.permissions.isAdmin {
+                Divider()
+                let muted = user.serverMuted
+                Button {
+                    session.setServerMute(user.id, muted: !muted, deafened: user.serverDeafened)
+                } label: {
+                    Label(muted ? "Unmute" : "Server Mute", systemImage: muted ? "mic" : "mic.slash")
+                }
+                Button {
+                    activeSheet = .permissions
+                } label: {
+                    Label("Permissions…", systemImage: "lock.shield")
+                }
+            }
+        }
+    }
+}
+
+private struct UserRowView: View {
+    let user: User
+    let isSelf: Bool
+
+    var body: some View {
+        HStack(spacing: 10) {
+            Image(systemName: user.selfMicMuted || user.serverMuted ? "mic.slash.fill" : "mic.fill")
+                .foregroundStyle(user.selfMicMuted || user.serverMuted ? .red : .green)
+                .imageScale(.small)
+                .accessibilityHidden(true)
+            VStack(alignment: .leading, spacing: 1) {
+                HStack(spacing: 4) {
+                    Text(user.nickname)
+                        .fontWeight(isSelf ? .semibold : .regular)
+                    if isSelf {
+                        Text("(you)")
+                            .font(.caption2)
+                            .foregroundStyle(.secondary)
+                    }
+                    if user.isGuest {
+                        Text("guest")
+                            .font(.caption2)
+                            .foregroundStyle(.secondary)
+                    }
+                }
+                if user.serverMuted || user.serverDeafened {
+                    Text(user.serverDeafened ? "server deafened" : "server muted")
+                        .font(.caption2)
+                        .foregroundStyle(.orange)
+                }
+            }
+            Spacer()
+            if user.selfDeafened {
+                Image(systemName: "headphones.slash")
+                    .imageScale(.small)
+                    .foregroundStyle(.secondary)
+                    .accessibilityHidden(true)
+            }
+        }
+        .accessibilityElement(children: .combine)
+        .accessibilityLabel("\(user.nickname)\(isSelf ? ", you" : "")\(user.isGuest ? ", guest" : "")\(user.selfMicMuted ? ", muted" : "")\(user.serverMuted ? ", server muted" : "")")
+    }
+}