Fix booking creation: localStorage token, conflict detection, paid_amount

- Fix api.ts getToken/saveToken to read from localStorage (matching AuthContext) — was reading sessionStorage causing all API requests to fail auth until refresh
- Fix session cleanup on 401 to use localStorage
- Add isConflict/availableRooms helpers to BookingModal; show amber warning when selected room is occupied with clickable free room suggestions
- Pass bookings prop to BookingModal via BookingCalendar
- Add paid_amount to backend POST /bookings INSERT
- Show alert() when booking creation fails so user sees the error
- Pass paidAmount in CalendarPage handleCreate

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/lib/api.ts

@@ -16,7 +16,7 @@ export class ApiError extends Error {
 
 function getToken(): string | null {
   try {
-    const s = sessionStorage.getItem('hotelsync-session')
+    const s = localStorage.getItem('hotelsync-session')
     return s ? (JSON.parse(s) as { token: string }).token : null
   } catch {
     return null
@@ -25,11 +25,11 @@ function getToken(): string | null {
 
 function saveToken(token: string) {
   try {
-    const s = sessionStorage.getItem('hotelsync-session')
+    const s = localStorage.getItem('hotelsync-session')
     if (!s) return
     const parsed = JSON.parse(s) as Record<string, unknown>
     parsed.token = token
-    sessionStorage.setItem('hotelsync-session', JSON.stringify(parsed))
+    localStorage.setItem('hotelsync-session', JSON.stringify(parsed))
   } catch {
     // ignore
   }
@@ -86,7 +86,7 @@ async function req<T>(
       saveToken(access_token)
       res = await doFetch(access_token)
     } else {
-      sessionStorage.removeItem('hotelsync-session')
+      localStorage.removeItem('hotelsync-session')
       window.location.href = '/login'
       throw new ApiError(401, 'Session expired')
     }