Add dark mode for PWA frontend

apps/frontend/src/components/BottomNav.css

@@ -65,9 +65,9 @@
   height: 3.5rem;
   border-radius: var(--radius-full);
   /* Stronger, brighter blue to make scan button easy to find */
-  background: #2b5bb5;
-  color: #ffffff;
-  box-shadow: 0 8px 26px rgba(43, 91, 181, 0.32);
+  background: var(--fab-bg);
+  color: var(--fab-text);
+  box-shadow: 0 8px 26px var(--fab-shadow);
   margin-top: -1.5rem;
   transition: transform 0.12s, box-shadow 160ms ease;
 }
@@ -78,12 +78,12 @@
 
 .nav-fab:hover {
   transform: translateY(-2px);
-  box-shadow: 0 12px 36px rgba(43, 91, 181, 0.36);
+  box-shadow: 0 12px 36px var(--fab-shadow);
 }
 
 .nav-fab:focus {
   outline: none;
-  box-shadow: 0 0 0 6px rgba(43,91,181,0.12), 0 8px 26px rgba(43, 91, 181, 0.32);
+  box-shadow: 0 0 0 6px rgba(43,91,181,0.12), 0 8px 26px var(--fab-shadow);
 }
 
 .nav-badge {

apps/frontend/src/components/LoginModal.css

@@ -118,7 +118,7 @@
 
 .modal-error {
   font-size: 0.82rem;
-  color: #b91c1c;
+  color: var(--error);
   margin: 0.25rem 0 0.75rem;
 }
 
@@ -149,7 +149,7 @@
 .modal-submit {
   background: var(--primary);
   border: none;
-  color: #fff;
+  color: var(--on-primary);
   padding: 0.55rem 1.35rem;
   border-radius: 999px;
   cursor: pointer;

apps/frontend/src/components/PharmacyList.css

@@ -139,8 +139,8 @@
 }
 
 .stock.low-stock {
-  background: rgba(180, 83, 9, 0.12);
-  color: #b45309;
+  background: var(--warning-container);
+  color: var(--warning);
 }
 
 .stock.out-of-stock {

apps/frontend/src/components/SavedNotifications.css

@@ -73,7 +73,7 @@
 }
 
 .saved-notifications-error {
-  color: #b91c1c;
+  color: var(--error);
 }
 
 .saved-notifications-list {
@@ -155,8 +155,8 @@
 }
 
 .saved-notifications-remove:hover:not(:disabled) {
-  border-color: #b91c1c;
-  color: #b91c1c;
+  border-color: var(--error);
+  color: var(--error);
   background: rgba(185, 28, 28, 0.06);
 }