feat: add keyboard shortcuts R/S/L/D/T/? with shortcuts modal

src/components/KeyboardShortcutsModal.tsx

@@ -0,0 +1,78 @@
+'use client';
+
+import { useEffect } from 'react';
+import { X } from 'lucide-react';
+
+interface KeyboardShortcutsModalProps {
+  open: boolean;
+  onClose: () => void;
+}
+
+const shortcuts = [
+  { key: 'R', description: 'Rectangle tool' },
+  { key: 'S', description: 'Span tool' },
+  { key: 'L', description: 'Line tool' },
+  { key: 'D', description: 'Delete tool' },
+  { key: 'T', description: 'Toggle theme (light/dark)' },
+  { key: 'Esc', description: 'Deselect tool / cancel action' },
+  { key: 'Del / ⌫', description: 'Delete selected annotation' },
+  { key: 'Enter', description: 'Edit selected span' },
+  { key: '1–6', description: 'Quick-assign span label (during span creation)' },
+  { key: '?', description: 'Show this help' },
+];
+
+export default function KeyboardShortcutsModal({ open, onClose }: KeyboardShortcutsModalProps) {
+  useEffect(() => {
+    if (!open) return;
+    const handleKey = (e: KeyboardEvent) => {
+      if (e.key === 'Escape' || e.key === '?') {
+        onClose();
+      }
+    };
+    window.addEventListener('keydown', handleKey);
+    return () => window.removeEventListener('keydown', handleKey);
+  }, [open, onClose]);
+
+  if (!open) return null;
+
+  return (
+    <div
+      className="fixed inset-0 z-50 flex items-center justify-center bg-black/50"
+      onClick={onClose}
+    >
+      <div
+        className="bg-background border border-border rounded-lg shadow-xl w-96 p-5"
+        onClick={(e) => e.stopPropagation()}
+      >
+        <div className="flex items-center justify-between mb-4">
+          <h2 className="text-sm font-semibold text-foreground">Keyboard Shortcuts</h2>
+          <button
+            onClick={onClose}
+            className="p-1 rounded text-muted-foreground hover:text-foreground hover:bg-secondary transition-colors"
+          >
+            <X className="w-4 h-4" />
+          </button>
+        </div>
+
+        <table className="w-full text-xs">
+          <tbody>
+            {shortcuts.map(({ key, description }) => (
+              <tr key={key} className="border-b border-border last:border-0">
+                <td className="py-1.5 pr-4 font-mono">
+                  <kbd className="px-1.5 py-0.5 bg-secondary rounded text-foreground font-mono text-[11px]">
+                    {key}
+                  </kbd>
+                </td>
+                <td className="py-1.5 text-muted-foreground">{description}</td>
+              </tr>
+            ))}
+          </tbody>
+        </table>
+
+        <p className="mt-4 text-[10px] text-muted-foreground text-center">
+          Press <kbd className="px-1 py-0.5 bg-secondary rounded font-mono">?</kbd> or <kbd className="px-1 py-0.5 bg-secondary rounded font-mono">Esc</kbd> to close
+        </p>
+      </div>
+    </div>
+  );
+}