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.

PROGRESS.md

@@ -10,6 +10,20 @@ up instantly. Newest status at the top.
 
 ## ▶ Where we left off / next action
 
+- **Done (2026-06-21):** **iOS iPhone-layout UX fixes.** (1) Channels are now a **drill-down**
+  on iPhone — new `ChannelBrowserView` (root list of top-level channels) → `ChannelDetailView`
+  (people in the channel + sub-channels + an explicit "Join Channel" button with password
+  prompt). The iPad 3-column `NavigationSplitView` is unchanged. (2) Extracted a self-contained
+  `UserRow` (context menu + sheets) from `UserListView` so admin actions are reused in the
+  drill-down. (3) Fixed the **off-screen chat compose box**: `MainView` now places
+  `VoiceControlsView` via `.safeAreaInset(edge: .bottom)` instead of a floating `.overlay`, so
+  it reserves layout space above the tab bar and cooperates with keyboard avoidance. (4)
+  **Collapsed Activity into Chat** like macOS/Windows: `ChatView` renders a merged, time-sorted
+  timeline of `messages` + `activityLog` (activity rows in gray); the separate Activity tab and
+  `ActivityLogView.swift` are removed. `xcodebuild` Debug for `generic/platform=iOS` BUILD
+  SUCCEEDED (sim slice still arm64-only → simulator run N/A). Next: on-device check of the
+  drill-down + compose box + unified timeline.
+
 - **Done (2026-06-21):** **Screen-audio sharing on macOS + iOS.** macOS uses ScreenCaptureKit
   (`ScreenAudioCapture.swift`) → `vc_stream_feed_pcm`; iOS uses a ReplayKit Broadcast Upload
   Extension (`VoiceCatBroadcast`) that forwards captured `.audioApp` PCM through a shared App