feat(design-system): reskin Charcoal — tokens dark-default + sidebar navy permanent

- Remplacement intégral index.css par palette Charcoal (DESIGN_SYSTEM.md v2.0)
- Dark = thème par défaut, .light = override via @custom-variant light
- Sidebar navy #0C1528 permanent (identique dark+light)
- Script anti-FOUC inline dans index.html
- Layout : radial-gradient sur <main>, sidebar 230px, max-w-[1100px]
- Renommage tokens Boréal→Charcoal sur ~45 composants
- Inversion dark: → baseline + light: sur primitives shadcn
- Fix logo blanc forcé dans sidebar
- ADR 006 mis à jour

Typecheck: OK · Tests: 122/122 ✅

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/shared/lib/theme.ts

@@ -14,11 +14,12 @@ const STORAGE_KEY = 'expria-theme'
 export function getInitialTheme(): Theme {
   const stored = localStorage.getItem(STORAGE_KEY)
   if (stored === 'light' || stored === 'dark') return stored
-  return window.matchMedia('(prefers-color-scheme: dark)').matches ? 'dark' : 'light'
+  if (window.matchMedia('(prefers-color-scheme: light)').matches) return 'light'
+  return 'dark'
 }
 
 export function applyTheme(theme: Theme): void {
-  document.documentElement.classList.toggle('dark', theme === 'dark')
+  document.documentElement.classList.toggle('light', theme === 'light')
 }
 
 export function persistTheme(theme: Theme): void {