feat: MVP-5 Sprint 2 - Command Palette and Global Shortcuts

- Add Command Palette (Ctrl+K / Cmd+K) with search and navigation
- Add global keyboard shortcuts: g h (dashboard), g n (notes), n (new note)
- Add keyboard shortcuts help dialog (?)
- Add shortcuts provider component
- Shortcuts ignore inputs/textareas for proper UX

src/hooks/use-global-shortcuts.ts

@@ -0,0 +1,65 @@
+import { useEffect, useState } from 'react'
+import { useRouter } from 'next/navigation'
+
+const SHORTCUTS: Record<string, string> = {
+  'g h': '/',        // go to dashboard
+  'g n': '/notes',  // go to notes
+  'n': '/new',      // new note
+  '/': '/notes',    // focus search (navigate to notes)
+  '?': 'show-help', // show shortcuts dialog
+}
+
+export function useGlobalShortcuts() {
+  const router = useRouter()
+  const [showHelp, setShowHelp] = useState(false)
+
+  useEffect(() => {
+    let keystroke = ''
+    let timeout: NodeJS.Timeout
+
+    const handleKeyDown = (e: KeyboardEvent) => {
+      // Ignore if in input/textarea/contenteditable
+      const target = e.target as HTMLElement
+      if (target.tagName === 'INPUT' ||
+          target.tagName === 'TEXTAREA' ||
+          target.isContentEditable) {
+        return
+      }
+
+      // Handle ? for help
+      if (e.key === '?' && !e.shiftKey) {
+        e.preventDefault()
+        setShowHelp(true)
+        return
+      }
+
+      // Track g + key combos
+      if (e.key === 'g' && !e.metaKey && !e.ctrlKey) {
+        keystroke = 'g'
+        clearTimeout(timeout)
+        timeout = setTimeout(() => { keystroke = '' }, 500)
+        return
+      }
+
+      if (keystroke === 'g') {
+        const combo = 'g ' + e.key
+        if (SHORTCUTS[combo]) {
+          e.preventDefault()
+          router.push(SHORTCUTS[combo])
+          keystroke = ''
+        }
+      }
+
+      // Direct shortcuts
+      if (e.key === 'n' && !e.metaKey && !e.ctrlKey) {
+        e.preventDefault()
+        router.push('/new')
+      }
+    }
+
+    window.addEventListener('keydown', handleKeyDown)
+    return () => window.removeEventListener('keydown', handleKeyDown)
+  }, [router])
+
+  return { showHelp, setShowHelp }
+}