feat: add persistent light/dark mode toggle

- Add CSS custom property token system to globals.css (dark + light variants)
- Add anti-FOUC inline script in layout.tsx head to apply stored theme before hydration
- Create useTheme hook (src/hooks/useTheme.ts) backed by usePersistentState
- Add Sun/Moon toggle button to LeftToolbar above Settings
- Wire useTheme into ThreePanelLayout and pass theme/onThemeToggle to LeftToolbar
- Migrate hardcoded hex values to CSS vars in LeftToolbar and ThreePanelLayout

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

app/globals.css

@@ -36,6 +36,67 @@
 @tailwind components;
 @tailwind utilities;
 
+:root,
+html[data-theme="dark"] {
+  color-scheme: dark;
+  --rah-bg-base: #0a0a0a;
+  --rah-bg-surface: #111111;
+  --rah-bg-subtle: #121212;
+  --rah-bg-panel: #141414;
+  --rah-bg-hover: #151515;
+  --rah-bg-elevated: #161616;
+  --rah-bg-active: #1a1a1a;
+  --rah-bg-modal: #141414;
+  --rah-border: #1a1a1a;
+  --rah-border-strong: #2a2a2a;
+  --rah-border-stronger: #3a3a3a;
+  --rah-scrollbar: #353535;
+  --rah-scrollbar-hover: #4a4a4a;
+  --rah-text-base: #e5e5e5;
+  --rah-text-secondary: #c7c7c7;
+  --rah-text-muted: #7a7a7a;
+  --rah-text-soft: #a3a3a3;
+  --rah-text-active: #f0f0f0;
+  --rah-text-inverse: #0a0a0a;
+  --rah-accent-green: #22c55e;
+  --rah-accent-green-hover: #16a34a;
+  --rah-accent-green-soft: rgba(34, 197, 94, 0.14);
+  --rah-accent-green-soft-strong: rgba(34, 197, 94, 0.22);
+  --rah-backdrop: rgba(0, 0, 0, 0.85);
+  --rah-shadow-modal: 0 8px 32px rgba(0, 0, 0, 0.8), 0 0 0 1px rgba(255, 255, 255, 0.05);
+  --rah-shadow-floating: 0 4px 12px rgba(0, 0, 0, 0.4);
+}
+
+html[data-theme="light"] {
+  color-scheme: light;
+  --rah-bg-base: #ffffff;
+  --rah-bg-surface: #fafafa;
+  --rah-bg-subtle: #f7f7f7;
+  --rah-bg-panel: #f5f5f5;
+  --rah-bg-hover: #f0f0f0;
+  --rah-bg-elevated: #ececec;
+  --rah-bg-active: #e8e8e8;
+  --rah-bg-modal: #ffffff;
+  --rah-border: #e5e5e5;
+  --rah-border-strong: #d4d4d4;
+  --rah-border-stronger: #bdbdbd;
+  --rah-scrollbar: #d4d4d4;
+  --rah-scrollbar-hover: #b5b5b5;
+  --rah-text-base: #111111;
+  --rah-text-secondary: #333333;
+  --rah-text-muted: #6b7280;
+  --rah-text-soft: #4b5563;
+  --rah-text-active: #000000;
+  --rah-text-inverse: #ffffff;
+  --rah-accent-green: #16a34a;
+  --rah-accent-green-hover: #15803d;
+  --rah-accent-green-soft: rgba(22, 163, 74, 0.12);
+  --rah-accent-green-soft-strong: rgba(22, 163, 74, 0.2);
+  --rah-backdrop: rgba(0, 0, 0, 0.5);
+  --rah-shadow-modal: 0 10px 36px rgba(0, 0, 0, 0.18), 0 0 0 1px rgba(0, 0, 0, 0.04);
+  --rah-shadow-floating: 0 6px 20px rgba(0, 0, 0, 0.16);
+}
+
 /* Modern Clean Design System - RA-H */
 * {
   margin: 0;
@@ -46,8 +107,8 @@
 html, body {
   height: 100%;
   overflow: hidden;
-  background-color: #0a0a0a;
-  color: #e5e5e5;
+  background-color: var(--rah-bg-base);
+  color: var(--rah-text-base);
   /* Geist Sans for optimal reading/writing experience */
   font-family: 'Geist', 'Inter', -apple-system, BlinkMacSystemFont, 'Segoe UI', sans-serif;
   font-size: 14px; 
@@ -71,21 +132,21 @@ html, body {
 }
 
 ::-webkit-scrollbar-track {
-  background: #0a0a0a;
+  background: var(--rah-bg-base);
 }
 
 ::-webkit-scrollbar-thumb {
-  background: #353535;
+  background: var(--rah-scrollbar);
   border-radius: 2px;
 }
 
 ::-webkit-scrollbar-thumb:hover {
-  background: #4a4a4a;
+  background: var(--rah-scrollbar-hover);
 }
 
 /* Utility classes */
 .border-divider {
-  border-color: #1a1a1a;
+  border-color: var(--rah-border);
 }
 
 .text-muted {