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>

ROADMAP.md

@@ -241,7 +241,7 @@ Underlay (PDF/DWF/DGN)
 | Plot ayarları (PlotSettings) | ✅ |
 | Çoklu named layout sekmeleri | ✅ |
 | VPLAYER — viewport katman override | 🔧 Altyapı var |
-| Layout Manager arayüzü | ⬜ |
+| Layout Manager arayüzü | ✅ |
 
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