feat(ui): selectable UI themes (10 palettes + swatch picker)

The UI was hardcoded to one Catppuccin Mocha palette (color literals in
view() + theme() pinned to Dark). It now sources colours from a chosen
theme's palette, selectable in Settings.

- theme.rs (NEW): Palette (13 semantic colour roles) + AppTheme enum
  (Catppuccin Mocha/Macchiato/Frappe/Latte, Dracula, Nord, Tokyo Night,
  Gruvbox Dark, Solarized Light, Gruvbox Light). Pure palette()/label()/
  ALL/base_theme()/is_dark() + WCAG relative_luminance()/contrast_ratio().
- config: theme: AppTheme field (serde-default Mocha) + backward-compat.
- app: theme() returns config.theme.base_theme() (iced widget chrome);
  view() + with_layout_picker() source colours from the palette; new
  ThemeSwatch canvas widget; a Theme section of clickable swatches in
  Settings; SelectTheme applies live + persists. Canvas widgets already
  take colours as data, so they re-theme for free.

Tests written alongside (+7 theme, +2 config; 135 -> 144 lib): every
palette clears WCAG AA text-on-base contrast (4.5:1) with subtext/accent
>= 3:1, is_dark matches luminance direction, variants distinct/labeled,
serde round-trips, default = Mocha.

Verified live: the Settings swatch grid renders all 10 palettes and the
whole UI re-themes (screenshot-checked Latte light + Dracula dark).
clippy --all-targets clean.

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

src/config.rs

@@ -1,3 +1,4 @@
+use crate::theme::AppTheme;
 use serde::{Deserialize, Serialize};
 use std::fs;
 use std::path::PathBuf;
@@ -137,6 +138,9 @@ pub struct AppConfig {
     /// Chosen arrangement of the in-call room screen.
     #[serde(default)]
     pub room_layout: RoomLayout,
+    /// Chosen UI colour theme.
+    #[serde(default)]
+    pub theme: AppTheme,
     #[serde(default)]
     pub custom_sound_self_join: Option<String>,
     #[serde(default)]
@@ -190,6 +194,7 @@ impl Default for AppConfig {
             controls_width: default_controls_width(),
             chat_drawer_width: default_chat_drawer_width(),
             room_layout: RoomLayout::default(),
+            theme: AppTheme::default(),
             custom_sound_self_join: None,
             custom_sound_peer_join: None,
             custom_sound_peer_leave: None,
@@ -318,6 +323,28 @@ mod tests {
         assert_eq!(back.window_y, Some(-50));
     }
 
+    #[test]
+    fn test_theme_field() {
+        // Default theme is Mocha (the original look).
+        assert_eq!(AppConfig::default().theme, AppTheme::Mocha);
+        // A chosen theme round-trips.
+        let cfg = AppConfig {
+            theme: AppTheme::Nord,
+            ..AppConfig::default()
+        };
+        let json = serde_json::to_string(&cfg).unwrap();
+        let back: AppConfig = serde_json::from_str(&json).unwrap();
+        assert_eq!(back.theme, AppTheme::Nord);
+    }
+
+    #[test]
+    fn old_config_without_theme_loads() {
+        // Configs predating the theme picker load with the default (Mocha).
+        let json = r#"{"input_device":"","output_device":"","noise_gate_threshold":0.01}"#;
+        let cfg: AppConfig = serde_json::from_str(json).unwrap();
+        assert_eq!(cfg.theme, AppTheme::Mocha);
+    }
+
     #[test]
     fn old_config_without_position_loads() {
         // A config written before window_x/window_y existed still deserializes