feat: add keyboard shortcuts and statistics dashboard

Keyboard Shortcuts:
- Create useKeyboardShortcuts hook with configurable shortcuts
- Support Ctrl/Cmd, Alt, Shift modifiers
- Add 7 global shortcuts (New note, Search, Quick capture, etc.)
- Create KeyboardShortcutsHelp modal with categorized shortcuts
- Show formatted shortcut keys (⌘ on Mac, Ctrl on Windows)
- Press ? to open shortcuts help

Statistics Dashboard:
- Create StatsDashboard component with productivity metrics
- Calculate productivity score (0-100) based on completion rate
- Show notes statistics (total, active, archived, priority)
- Display todos statistics with completion rate
- Track content stats (notes with todos/images/files, unique tags)
- Show recent activity (notes updated in last 7 days)
- Visual progress bar for productivity score
- Integrate dashboard in left column below calendar

src/hooks/useKeyboardShortcuts.ts

@@ -0,0 +1,87 @@
+import { useEffect } from 'react';
+
+export interface KeyboardShortcut {
+  key: string;
+  ctrl?: boolean;
+  alt?: boolean;
+  shift?: boolean;
+  meta?: boolean;
+  description: string;
+  handler: () => void;
+  preventDefault?: boolean;
+}
+
+/**
+ * Custom hook for managing keyboard shortcuts
+ */
+export function useKeyboardShortcuts(
+  shortcuts: KeyboardShortcut[],
+  enabled = true
+) {
+  useEffect(() => {
+    if (!enabled) return;
+
+    const handleKeyDown = (event: KeyboardEvent) => {
+      for (const shortcut of shortcuts) {
+        const keyMatch = event.key.toLowerCase() === shortcut.key.toLowerCase();
+        const ctrlMatch = shortcut.ctrl ? (event.ctrlKey || event.metaKey) : !event.ctrlKey && !event.metaKey;
+        const altMatch = shortcut.alt ? event.altKey : !event.altKey;
+        const shiftMatch = shortcut.shift ? event.shiftKey : !event.shiftKey;
+        const metaMatch = shortcut.meta ? event.metaKey : !event.metaKey;
+
+        if (keyMatch && ctrlMatch && altMatch && shiftMatch && metaMatch) {
+          if (shortcut.preventDefault !== false) {
+            event.preventDefault();
+          }
+          shortcut.handler();
+          break;
+        }
+      }
+    };
+
+    window.addEventListener('keydown', handleKeyDown);
+    return () => window.removeEventListener('keydown', handleKeyDown);
+  }, [shortcuts, enabled]);
+}
+
+/**
+ * Format keyboard shortcut for display
+ */
+export function formatShortcut(shortcut: KeyboardShortcut): string {
+  const parts: string[] = [];
+
+  if (shortcut.ctrl || shortcut.meta) {
+    parts.push(navigator.platform.includes('Mac') ? '⌘' : 'Ctrl');
+  }
+  if (shortcut.alt) {
+    parts.push(navigator.platform.includes('Mac') ? '⌥' : 'Alt');
+  }
+  if (shortcut.shift) {
+    parts.push(navigator.platform.includes('Mac') ? '⇧' : 'Shift');
+  }
+
+  parts.push(shortcut.key.toUpperCase());
+
+  return parts.join('+');
+}
+
+/**
+ * Common keyboard shortcuts used across the app
+ */
+export const COMMON_SHORTCUTS = {
+  NEW_NOTE: { key: 'n', ctrl: true, description: 'Nouvelle note' },
+  SEARCH: { key: 'k', ctrl: true, description: 'Rechercher' },
+  QUICK_CAPTURE: { key: 'q', ctrl: true, description: 'Capture rapide' },
+  TOGGLE_SIDEBAR: { key: 'b', ctrl: true, description: 'Basculer sidebar' },
+  SAVE: { key: 's', ctrl: true, description: 'Sauvegarder' },
+  CLOSE: { key: 'Escape', description: 'Fermer' },
+  HELP: { key: '?', shift: true, description: 'Aide raccourcis' },
+  ARCHIVE: { key: 'e', ctrl: true, description: 'Archiver note' },
+  DELETE: { key: 'Delete', ctrl: true, description: 'Supprimer' },
+  NEXT: { key: 'j', description: 'Suivant' },
+  PREVIOUS: { key: 'k', description: 'Précédent' },
+  TOGGLE_TODO: { key: 'Enter', ctrl: true, description: 'Toggle todo' },
+  BOLD: { key: 'b', ctrl: true, description: 'Gras' },
+  ITALIC: { key: 'i', ctrl: true, description: 'Italique' },
+  LINK: { key: 'k', ctrl: true, shift: true, description: 'Lien' },
+};