feat(ui): add tab navigation system for desktop and mobile

Desktop (2 tabs):
- Status tab: User Stats + Info Box + Character Thoughts
- Inventory tab: Inventory system (dedicated space)

Mobile (3 tabs):
- Stats tab: User Stats only
- Info tab: Info Box + Character Thoughts
- Inventory tab: Inventory only

Features:
- Created desktop.js module for desktop tab management
- Updated mobile.js to use 3-tab structure (more breathing room on small screens)
- Added CSS styling for desktop tabs (hover states, active indicators)
- Implemented viewport transition handlers (desktop ↔ mobile)
- Tabs replace dividers (cleaner visual separation)
- Character thoughts can now expand to fill vertical space

This resolves the cramped 4-section panel issue by organizing content into logical tabs on both desktop and mobile.

index.js

@@ -100,6 +100,10 @@ import {
     setupMobileKeyboardHandling,
     setupContentEditableScrolling
 } from './src/systems/ui/mobile.js';
+import {
+    setupDesktopTabs,
+    removeDesktopTabs
+} from './src/systems/ui/desktop.js';
 
 // Feature modules
 import { setupPlotButtons, sendPlotProgression } from './src/systems/features/plotProgression.js';
@@ -368,6 +372,11 @@ async function initUI() {
     // Setup mobile toggle button
     setupMobileToggle();
 
+    // Setup desktop tabs (only on desktop viewport)
+    if (window.innerWidth > 1000) {
+        setupDesktopTabs();
+    }
+
     // Setup collapse/expand toggle button
     setupCollapseToggle();