feat: COLORSCHEME command — runtime UI theme/color-scheme switching

Adds active_theme: Theme field to app state and wires the daemon's
.theme() callback to use it dynamically. COLORSCHEME <name> switches
among all 19 built-in iced themes (Dark, Light, Dracula, Nord, Tokyo
Night, Gruvbox, Kanagawa, Moonfly, Nightfly, etc.).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/update.rs

@@ -2365,6 +2365,11 @@ impl H7CAD {
                 Task::none()
             }
 
+            Message::SetTheme(theme) => {
+                self.active_theme = theme;
+                Task::none()
+            }
+
             Message::ViewportContextMenuClose => {
                 let i = self.active_tab;
                 self.tabs[i].scene.selection.borrow_mut().context_menu = None;