feat: design system overhaul — sidebar, AI chats, settings, brainstorm, color cleanup

- Sidebar: dynamic brand-accent colors, brainstorm section restyled
- AI chat general: popup panel with expand/collapse, hides when contextual AI open
- AI chat contextual: tabs reordered (Actions first), X close button, height fix
- Settings: all tabs restyled, 6 new color presets (sage, terracotta, iron, etc.)
- Global color cleanup: emerald/orange hardcoded → brand-accent dynamic
- Brainstorm page: orange → brand-accent throughout
- PageEntry animation component added to key pages
- Floating AI button: bg-brand-accent instead of hardcoded black
- i18n: all 15 locales updated with new AI/billing keys
- Billing: freemium quota tracking, BYOK, stripe subscription scaffolding
- Admin: integrated into new design
- AGENTS.md + CLAUDE.md project rules added

memento-note/scripts/merge-missing-locale-keys.mjs

@@ -0,0 +1,35 @@
+/**
+ * Ajoute récursivement les clés présentes dans locales/en.json mais absentes
+ * des autres fichiers (valeur = texte anglais, à retraduire si besoin).
+ * Usage: node scripts/merge-missing-locale-keys.mjs
+ */
+import fs from 'fs'
+import path from 'path'
+import { fileURLToPath } from 'url'
+
+const __dirname = path.dirname(fileURLToPath(import.meta.url))
+const localesDir = path.join(__dirname, '..', 'locales')
+const enPath = path.join(localesDir, 'en.json')
+
+function mergeMissing(target, source) {
+  for (const [k, v] of Object.entries(source)) {
+    if (v !== null && typeof v === 'object' && !Array.isArray(v)) {
+      if (!target[k] || typeof target[k] !== 'object' || Array.isArray(target[k])) {
+        target[k] = {}
+      }
+      mergeMissing(target[k], v)
+    } else {
+      if (!(k in target)) target[k] = v
+    }
+  }
+}
+
+const en = JSON.parse(fs.readFileSync(enPath, 'utf8'))
+for (const name of fs.readdirSync(localesDir)) {
+  if (!name.endsWith('.json') || name === 'en.json') continue
+  const p = path.join(localesDir, name)
+  const loc = JSON.parse(fs.readFileSync(p, 'utf8'))
+  mergeMissing(loc, en)
+  fs.writeFileSync(p, JSON.stringify(loc, null, 2) + '\n')
+}
+console.log('merge-missing-locale-keys: OK')