feat(ui): add navigation header and wire up real page routes

- Add sticky header with book management nav links and logout button
- Replace placeholder page components with real feature imports
- Add login page route to router
- Fix date-time formatting for consistent YYYY-MM-DD HH:mm output

src/lib/formatters.ts

@@ -1,5 +1,11 @@
 export function formatDateTime(iso: string): string {
-  return new Date(iso).toLocaleString()
+  const d = new Date(iso)
+  const Y = d.getFullYear()
+  const M = String(d.getMonth() + 1).padStart(2, "0")
+  const D = String(d.getDate()).padStart(2, "0")
+  const h = String(d.getHours()).padStart(2, "0")
+  const m = String(d.getMinutes()).padStart(2, "0")
+  return `${Y}-${M}-${D} ${h}:${m}`
 }
 
 export function formatDate(iso: string): string {