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/components/settings/usage-breakdown.tsx

@@ -0,0 +1,99 @@
+'use client';
+
+import { useQuery } from '@tanstack/react-query';
+import { Loader2, BarChart2 } from 'lucide-react';
+import { useLanguage } from '@/lib/i18n';
+import { cn } from '@/lib/utils';
+
+interface QuotaEntry {
+  remaining: number;
+  limit: number;
+  used: number;
+}
+
+type Quotas = Record<string, QuotaEntry>;
+
+const FEATURE_LABEL_KEYS: Record<string, string> = {
+  aiSummary: 'sidebar.aiSummary',
+  aiFlashcards: 'sidebar.aiFlashcards',
+  aiMindmap: 'sidebar.aiMindmap',
+  aiTranscribe: 'sidebar.aiTranscribe',
+  aiDiagram: 'sidebar.aiDiagram',
+  aiAgent: 'sidebar.aiAgent',
+};
+
+function UsageBar({ used, limit, isUnlimited }: { used: number; limit: number; isUnlimited: boolean }) {
+  const pct = isUnlimited ? 0 : Math.min(100, limit > 0 ? (used / limit) * 100 : 0);
+  const color =
+    pct >= 90 ? 'bg-rose-500' : pct >= 70 ? 'bg-amber-500' : 'bg-emerald-500';
+
+  return (
+    <div className="h-1.5 w-full rounded-full bg-foreground/10 overflow-hidden">
+      {!isUnlimited && (
+        <div
+          className={cn('h-full rounded-full transition-all', color)}
+          style={{ width: `${pct}%` }}
+        />
+      )}
+    </div>
+  );
+}
+
+export function UsageBreakdown() {
+  const { t } = useLanguage();
+
+  const { data, isLoading } = useQuery<{ quotas: Quotas }>({
+    queryKey: ['usage', 'current'],
+    queryFn: async () => {
+      const res = await fetch('/api/usage/current');
+      if (!res.ok) throw new Error('Failed to fetch usage');
+      return res.json();
+    },
+  });
+
+  const quotas = data?.quotas ?? {};
+  const entries = Object.entries(quotas);
+
+  return (
+    <div className="rounded-xl border border-border/40 bg-paper p-6 space-y-4">
+      <div className="flex items-center gap-2">
+        <BarChart2 className="h-4 w-4 text-muted-foreground" />
+        <h3 className="text-sm font-semibold text-foreground">
+          {t('billing.usageThisPeriod')}
+        </h3>
+      </div>
+
+      {isLoading ? (
+        <div className="flex justify-center py-4">
+          <Loader2 className="h-5 w-5 animate-spin text-muted-foreground" />
+        </div>
+      ) : entries.length === 0 ? (
+        <p className="text-xs text-muted-foreground py-2">{t('billing.noUsage')}</p>
+      ) : (
+        <div className="space-y-3">
+          {entries.map(([feature, quota]) => {
+            const isUnlimited = quota.limit === -1 || !isFinite(quota.limit);
+            const labelKey = FEATURE_LABEL_KEYS[feature];
+            const label = labelKey ? t(labelKey) : feature;
+
+            return (
+              <div key={feature} className="space-y-1">
+                <div className="flex items-center justify-between">
+                  <span className="text-xs text-muted-foreground">{label}</span>
+                  <span className="text-xs font-medium text-foreground tabular-nums">
+                    {isUnlimited ? (
+                      <span className="text-emerald-600 dark:text-emerald-400">{t('billing.unlimited')}</span>
+                    ) : (
+                      `${quota.used} / ${quota.limit}`
+                    )}
+                  </span>
+                </div>
+                <UsageBar used={quota.used} limit={quota.limit} isUnlimited={isUnlimited} />
+              </div>
+            );
+          })}
+        </div>
+      )}
+    </div>
+  );
+}