feat(ui): selectable room layouts with a thumbnail picker

Add three in-call room layouts — 3-Column (Participants | Chat | Controls),
Bottom Dock (Participants+Controls over a full-width Chat strip), and Drawer
(Participants | Controls with a collapsible Chat panel) — chosen via one
persisted RoomLayout config setting and applied live.

Picker UX: a square layout button (drawn LayoutIcon glyph) in the top bar of the
launch and in-call screens opens a popup gallery (dimmed click-to-dismiss
backdrop + centered panel) of clickable schematic thumbnails; the Settings screen
shows the same thumbnails inline (no button). Thumbnails are drawn with the
canvas widget (new LayoutThumb program — colored panel boxes, blue border on the
selected one), so no image-decoding dependency is added.

Each layout's panel boundaries are draggable (DividerKind gains Controls +
ChatDrawer for the 3-column right divider and the drawer's left edge; new
clamp_controls_width / clamp_chat_drawer_width, persisted + re-clamped on resize).
Participants width is shared across layouts but capped per layout at render time
so a fixed panel can't starve the Fill panel (e.g. a wide Participants width set
in the dock layout won't collapse Chat in 3-column or Controls in the drawer).
The Drawer layout adds a header chat-toggle. +1 clamp test (now 127 tests).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/config.rs

@@ -25,6 +25,34 @@ impl NetworkMode {
         [NetworkMode::RelayNoDiscovery, NetworkMode::N0Full, NetworkMode::DirectOnly];
 }
 
+/// Arrangement of the in-call room screen, chosen via the layout picker.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Serialize, Deserialize, Default)]
+pub enum RoomLayout {
+    /// Participants | Chat | Controls, all columns visible at once.
+    ThreeColumn,
+    /// Participants + Controls on top, full-width Chat docked along the bottom.
+    #[default]
+    BottomDock,
+    /// Participants | Controls, with a collapsible Chat drawer on the right edge.
+    Drawer,
+}
+
+impl RoomLayout {
+    /// All variants, in picker display order.
+    pub const ALL: [RoomLayout; 3] =
+        [RoomLayout::ThreeColumn, RoomLayout::BottomDock, RoomLayout::Drawer];
+}
+
+impl std::fmt::Display for RoomLayout {
+    fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
+        f.write_str(match self {
+            RoomLayout::ThreeColumn => "3-Column",
+            RoomLayout::BottomDock => "Bottom Dock",
+            RoomLayout::Drawer => "Drawer",
+        })
+    }
+}
+
 impl std::fmt::Display for NetworkMode {
     fn fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result {
         let label = match self {
@@ -56,6 +84,14 @@ fn default_chat_height() -> f32 {
     180.0
 }
 
+fn default_controls_width() -> f32 {
+    280.0
+}
+
+fn default_chat_drawer_width() -> f32 {
+    320.0
+}
+
 #[derive(Debug, Clone, Serialize, Deserialize, PartialEq)]
 pub struct AppConfig {
     /// Last nickname used to join/create a room; pre-filled on the launch screen.
@@ -84,6 +120,15 @@ pub struct AppConfig {
     pub participants_width: f32,
     #[serde(default = "default_chat_height")]
     pub chat_height: f32,
+    /// Controls panel width for the 3-column layout (px).
+    #[serde(default = "default_controls_width")]
+    pub controls_width: f32,
+    /// Chat drawer width for the drawer layout (px).
+    #[serde(default = "default_chat_drawer_width")]
+    pub chat_drawer_width: f32,
+    /// Chosen arrangement of the in-call room screen.
+    #[serde(default)]
+    pub room_layout: RoomLayout,
     #[serde(default)]
     pub custom_sound_self_join: Option<String>,
     #[serde(default)]
@@ -116,6 +161,9 @@ impl Default for AppConfig {
             notifications_enabled: true,
             participants_width: default_participants_width(),
             chat_height: default_chat_height(),
+            controls_width: default_controls_width(),
+            chat_drawer_width: default_chat_drawer_width(),
+            room_layout: RoomLayout::default(),
             custom_sound_self_join: None,
             custom_sound_peer_join: None,
             custom_sound_peer_leave: None,
@@ -186,6 +234,10 @@ mod tests {
         assert_eq!(deserialized.chat_height, 180.0);
         // Configs predating the remembered username load the default nickname.
         assert_eq!(deserialized.username, "Peer");
+        // Configs predating the layout picker load the default layout + sizes.
+        assert_eq!(deserialized.room_layout, RoomLayout::BottomDock);
+        assert_eq!(deserialized.controls_width, 280.0);
+        assert_eq!(deserialized.chat_drawer_width, 320.0);
         assert!(deserialized.custom_sound_self_join.is_none());
         assert!(deserialized.custom_sound_peer_join.is_none());
         assert!(deserialized.custom_sound_peer_leave.is_none());