refactor: move discounts into admin panel as component, rebuild modal with proper field handling

- DiscountsManagement component integrated into admin/+page.svelte
- Removed standalone /admin/discounts route
- Modal rebuilt with toggle buttons for campaign type, conditional field groups
- Milestone unit auto-restricted based on milestone type (bookings-only for count types, months/years for anniversary)
- Mobile-first: card layout on mobile, table on desktop (md: breakpoint)
- Native select elements instead of broken bits-ui Select components

frontend/src/routes/admin/+page.svelte

@@ -11,6 +11,7 @@
 	import HolidayHours from '$lib/components/admin/HolidayHours.svelte';
 	import WeeklySchedule from '$lib/components/admin/WeeklySchedule.svelte';
 	import ServicesManagement from '$lib/components/admin/ServicesManagement.svelte';
+	import DiscountsManagement from '$lib/components/admin/DiscountsManagement.svelte';
 	import UserModal from '$lib/components/admin/UserModal.svelte';
 	import BookingModal from '$lib/components/admin/BookingModal.svelte';
 
@@ -209,6 +210,48 @@
 				</table>
 			</div>
 		</div>
+
+		<!-- Discounts Management Card Skeleton -->
+		<div class="rounded-lg border p-6">
+			<div class="mb-4 flex flex-col gap-3 sm:flex-row sm:items-center sm:justify-between">
+				<div class="space-y-2">
+					<Skeleton class="h-6 w-40" />
+					<Skeleton class="h-4 w-64" />
+				</div>
+				<Skeleton class="h-10 w-32" />
+			</div>
+			<div class="mt-4 hidden w-full overflow-x-auto md:block">
+				<table class="w-full table-auto border-collapse text-sm">
+					<thead>
+						<tr class="border-b text-left text-xs text-gray-500">
+							<th class="py-3"><Skeleton class="h-4 w-24" /></th>
+							<th class="py-3"><Skeleton class="h-4 w-20" /></th>
+							<th class="py-3"><Skeleton class="h-4 w-16" /></th>
+							<th class="py-3"><Skeleton class="h-4 w-16" /></th>
+							<th class="py-3"><Skeleton class="h-4 w-16" /></th>
+							<th class="py-3"><Skeleton class="h-4 w-40" /></th>
+						</tr>
+					</thead>
+					<tbody>
+						{#each Array(2) as _, i (i)}
+							<tr class="border-b">
+								<td class="py-3"><Skeleton class="h-4 w-32" /></td>
+								<td class="py-3"><Skeleton class="h-4 w-24" /></td>
+								<td class="py-3"><Skeleton class="h-4 w-12" /></td>
+								<td class="py-3"><Skeleton class="h-4 w-16" /></td>
+								<td class="py-3"><Skeleton class="h-4 w-12" /></td>
+								<td class="py-3">
+									<div class="flex justify-end gap-2">
+										<Skeleton class="h-8 w-16" />
+										<Skeleton class="h-8 w-16" />
+									</div>
+								</td>
+							</tr>
+						{/each}
+					</tbody>
+				</table>
+			</div>
+		</div>
 	</div>
 {:else if pageState === 'authorized'}
 	<div class="mx-auto max-w-6xl space-y-6 p-6">
@@ -227,6 +270,7 @@
 		<HolidayHours />
 		<WeeklySchedule />
 		<ServicesManagement />
+		<DiscountsManagement />
 	</div>
 
 	<!-- Modals -->