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/components/KeyboardShortcutsHelp.tsx

@@ -0,0 +1,82 @@
+import { Dialog, DialogContent, DialogHeader, DialogTitle, DialogDescription } from "@/components/ui/dialog";
+import { Badge } from "@/components/ui/badge";
+import { ScrollArea } from "@/components/ui/scroll-area";
+import { Keyboard } from "lucide-react";
+import { type KeyboardShortcut, formatShortcut } from "@/hooks/useKeyboardShortcuts";
+
+interface KeyboardShortcutsHelpProps {
+  open: boolean;
+  onOpenChange: (open: boolean) => void;
+  shortcuts: KeyboardShortcut[];
+}
+
+export function KeyboardShortcutsHelp({ open, onOpenChange, shortcuts }: KeyboardShortcutsHelpProps) {
+  // Group shortcuts by category
+  const groupedShortcuts = shortcuts.reduce((acc, shortcut) => {
+    // Determine category based on description
+    let category = 'Général';
+
+    if (shortcut.description.toLowerCase().includes('note')) {
+      category = 'Notes';
+    } else if (shortcut.description.toLowerCase().includes('recherch')) {
+      category = 'Recherche';
+    } else if (shortcut.description.toLowerCase().includes('todo') || shortcut.description.toLowerCase().includes('tâche')) {
+      category = 'Tâches';
+    } else if (shortcut.description.toLowerCase().includes('navigation')) {
+      category = 'Navigation';
+    } else if (shortcut.description.toLowerCase().includes('édition') || shortcut.description.toLowerCase().includes('gras') || shortcut.description.toLowerCase().includes('italique')) {
+      category = 'Édition';
+    }
+
+    if (!acc[category]) {
+      acc[category] = [];
+    }
+    acc[category].push(shortcut);
+    return acc;
+  }, {} as Record<string, KeyboardShortcut[]>);
+
+  return (
+    <Dialog open={open} onOpenChange={onOpenChange}>
+      <DialogContent className="max-w-2xl">
+        <DialogHeader>
+          <DialogTitle className="flex items-center gap-2">
+            <Keyboard className="h-5 w-5" />
+            Raccourcis clavier
+          </DialogTitle>
+          <DialogDescription>
+            Utilisez ces raccourcis pour naviguer plus rapidement
+          </DialogDescription>
+        </DialogHeader>
+
+        <ScrollArea className="h-[500px] pr-4">
+          <div className="space-y-6">
+            {Object.entries(groupedShortcuts).map(([category, categoryShortcuts]) => (
+              <div key={category}>
+                <h3 className="text-sm font-semibold text-muted-foreground uppercase tracking-wide mb-3">
+                  {category}
+                </h3>
+                <div className="space-y-2">
+                  {categoryShortcuts.map((shortcut, index) => (
+                    <div
+                      key={index}
+                      className="flex items-center justify-between py-2 px-3 rounded-lg hover:bg-accent transition-colors"
+                    >
+                      <span className="text-sm">{shortcut.description}</span>
+                      <Badge variant="secondary" className="font-mono text-xs">
+                        {formatShortcut(shortcut)}
+                      </Badge>
+                    </div>
+                  ))}
+                </div>
+              </div>
+            ))}
+          </div>
+        </ScrollArea>
+
+        <div className="text-xs text-muted-foreground text-center pt-4 border-t">
+          Appuyez sur <Badge variant="outline" className="mx-1">?</Badge> pour afficher cette aide
+        </div>
+      </DialogContent>
+    </Dialog>
+  );
+}