polish(ui): tooltip on the layout button + even Audio Devices spacing

- The top-bar room-layout button was an icon-only canvas with no label or
  tooltip, while the Settings button beside it is labeled — a discoverability
  and consistency gap. Wrap it in the existing tooltip pattern ("Room layout",
  Position::Bottom) so its purpose is discoverable on hover.
- The Audio Devices input/output columns used spacing(4) while every other
  Settings section uses spacing(8); bump both to 8 for an even vertical rhythm.

Pure presentational changes. Build + clippy clean.

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

src/app/mod.rs

@@ -1000,14 +1000,21 @@ fn view(state: &AppState) -> Element<'_, AppMessage> {
 
     let top_bar = row![
         horizontal_space(),
-        button(
-            Canvas::new(LayoutIcon { fg: color_text })
-                .width(iced::Length::Fixed(18.0))
-                .height(iced::Length::Fixed(18.0))
+        tooltip(
+            button(
+                Canvas::new(LayoutIcon { fg: color_text })
+                    .width(iced::Length::Fixed(18.0))
+                    .height(iced::Length::Fixed(18.0))
+            )
+            .on_press(AppMessage::OpenLayoutPicker)
+            .style(b_style(color_surface, color_blue, color_text, 6.0))
+            .padding(8),
+            container(text("Room layout").size(11).color(color_text))
+                .padding(8)
+                .style(c_style(color_crust, color_surface, 6.0)),
+            iced::widget::tooltip::Position::Bottom,
         )
-        .on_press(AppMessage::OpenLayoutPicker)
-        .style(b_style(color_surface, color_blue, color_text, 6.0))
-        .padding(8),
+        .gap(8),
         button(
             row![
                 icon(IconKind::Settings, 15.0, color_text),
@@ -1222,7 +1229,7 @@ fn view(state: &AppState) -> Element<'_, AppMessage> {
                         slider(0.0..=2.0, state.config.input_volume, AppMessage::InputVolumeChanged)
                             .step(0.05)
                             .on_release(AppMessage::PersistConfig),
-                    ].spacing(4).width(iced::Length::Fill),
+                    ].spacing(8).width(iced::Length::Fill),
                     column![
                         text("Output Device").size(12).color(color_subtext),
                         pick_list(
@@ -1234,7 +1241,7 @@ fn view(state: &AppState) -> Element<'_, AppMessage> {
                         slider(0.0..=2.0, state.config.output_volume, AppMessage::OutputVolumeChanged)
                             .step(0.05)
                             .on_release(AppMessage::PersistConfig),
-                    ].spacing(4).width(iced::Length::Fill),
+                    ].spacing(8).width(iced::Length::Fill),
                 ].spacing(20).align_y(iced::alignment::Vertical::Top).width(iced::Length::Fill),
                 vertical_space(section_gap),