feat(admin): resource-mode booking flow with a hold countdown

The engine from tasks 06 and 07 could find slots and hold them, but nothing in
the panel could actually book one.

- Search, hold, confirm stay three separate steps because they are three
  separate states: between seeing a slot and taking it the seat is still open,
  and between taking and confirming there is a deadline
- HoldCountdown reads the server's expires_at rather than starting its own
  timer at render: browser clock skew and network latency both cost seconds,
  and those seconds are exactly where a hold is lost. It turns urgent under a
  minute and tells the parent the moment it lapses
- Per-role resource swap offers only the resources the engine returned for that
  same slot. Listing every resource in the branch would let an operator pick
  one that was never free and collect a 409
- An empty result is not an error: the reason code renders as a sentence
  saying what to change
- Confirm requires a doctor and stays disabled until one is chosen — the
  endpoint rejects it anyway, and finding that out after the hold clock has
  been running is the wrong time

Reached from the appointments page as a separate action rather than folded into
the existing form: its search comes from the intersection of resource
calendars, not from one doctor's slots, and merging the two would confuse both.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

assets/admin/pages/AppointmentsPage.tsx

@@ -795,6 +795,18 @@ export default function AppointmentsPage() {
             <AdjustmentsHorizontalIcon style={{ width: 16 }} />
           </button>
 
+          {/* رزرو منبع‌محور مسیر جداست چون جستجویش از تقاطع تقویم منابع می‌آید، نه از
+              اسلات‌های یک پزشک؛ ادغامشان در یک فرم، هر دو را گیج می‌کرد. */}
+          {!isRepresentation && canCreateAppt && (
+            <button
+              className="btn secondary sm"
+              onClick={() => navigate('/admin/resource-booking')}
+              style={{ display: 'flex', alignItems: 'center', gap: 5 }}
+            >
+              نوبت منبع‌محور
+            </button>
+          )}
+
           {!isRepresentation && canCreateAppt && (
             <button
               className="btn primary sm"