style: change partial charge link color from blue to gray

Changed the 'Partial charge' toggle link color from blue to gray to
match the overall design aesthetic.

frontend/src/lib/components/payments/PaymentModal.svelte

@@ -276,6 +276,7 @@
 	let lastValidCardAmount = $state<string>('');
 	let savedCardPaymentAmount = $state<string>('');
 	let lastValidSavedCardAmount = $state<string>('');
+	let showPartialCharge = $state(false);
 
 	let pollingInterval: ReturnType<typeof setInterval> | null = null;
 
@@ -1539,7 +1540,16 @@
 					</div>
 				{/if}
 
-				<div>
+				<div class="border-t pt-3">
+					<button
+						type="button"
+						onclick={() => (showPartialCharge = !showPartialCharge)}
+						class="text-sm text-gray-600 hover:text-gray-800 hover:underline"
+					>
+						{showPartialCharge ? '− Hide partial charge' : '+ Partial charge'}
+					</button>
+					{#if showPartialCharge}
+						<div class="mt-3">
 							<label for="card-amount" class="text-sm font-medium text-gray-700">
 								Amount to Charge
 							</label>
@@ -1564,6 +1574,8 @@
 								Leave blank to charge full amount ({formatCurrency(totalDue)})
 							</p>
 						</div>
+					{/if}
+				</div>
 
 				<div class="flex gap-3">
 					<Button variant="ghost" onclick={resetToSelect} class="min-h-11 flex-1">Back</Button>
@@ -1834,7 +1846,16 @@
 				/>
 
 				{#if selectedSavedCardId}
-					<div>
+					<div class="border-t pt-3">
+						<button
+							type="button"
+							onclick={() => (showPartialCharge = !showPartialCharge)}
+							class="text-sm text-gray-600 hover:text-gray-800 hover:underline"
+						>
+							{showPartialCharge ? '− Hide partial charge' : '+ Partial charge'}
+						</button>
+						{#if showPartialCharge}
+							<div class="mt-3">
 								<label for="saved-card-amount" class="text-sm font-medium text-gray-700">
 									Amount to Charge
 								</label>
@@ -1860,6 +1881,8 @@
 								</p>
 							</div>
 						{/if}
+					</div>
+				{/if}
 
 				<div class="flex gap-3">
 					<Button variant="ghost" onclick={resetToSelect} class="min-h-11 flex-1">Back</Button>