refactor(admin): compact UI styling for data-dense admin interface

- Reduce table cell padding (px-5 py-4 → px-3 py-2) across OrderTableBody, ProductTableBody, StopTableBody, AdminTable
- Narrow sidebar (w-60 → w-56) with smaller nav items
- Reduce page/card padding in admin-design-system.css
- Smaller default buttons (md → sm) in AdminButton
- Smaller page headers (text-2xl → text-xl, w-12 → w-10 icon)
- Tighter cards and filter tabs

Makes admin UI feel like professional B2B tool at 100% zoom

src/styles/admin-design-system.css

@@ -41,9 +41,9 @@
   --admin-sidebar-active: #5c584f;    /* between */
   
   /* Spacing */
-  --admin-page-padding: 1.5rem;      /* px-6 */
-  --admin-section-gap: 1rem;         /* gap-4 */
-  --admin-card-padding: 1.25rem;     /* p-5 */
+  --admin-page-padding: 1rem;         /* px-4 - reduced for density */
+  --admin-section-gap: 0.75rem;       /* gap-3 - reduced */
+  --admin-card-padding: 1rem;         /* p-4 - reduced */
   
   /* Border Radius */
   --admin-radius-sm: 0.5rem;         /* rounded-lg */
@@ -102,7 +102,7 @@
 .admin-table {
   width: 100%;
   text-align: left;
-  font-size: 0.875rem; /* text-sm */
+  font-size: 0.8125rem; /* 13px - reduced for density */
 }
 
 .admin-table thead {
@@ -111,8 +111,8 @@
 }
 
 .admin-table th {
-  padding: 0.875rem 1.25rem; /* py-3.5 px-5 */
-  font-size: 0.625rem; /* text-[10px] */
+  padding: 0.75rem 1rem;             /* py-3 px-4 - reduced for density */
+  font-size: 0.5625rem;              /* text-9px - reduced */
   font-weight: 700;
   text-transform: uppercase;
   letter-spacing: 0.1em;
@@ -137,7 +137,8 @@
 }
 
 .admin-table td {
-  padding: 0.875rem 1.25rem; /* py-3.5 px-5 */
+  padding: 0.625rem 1rem;            /* py-2.5 px-4 - reduced for density */
+  font-size: 0.8125rem;              /* 13px - reduced */
 }
 
 /* === Admin Stats Bar === */
@@ -184,10 +185,10 @@
 .admin-btn {
   display: inline-flex;
   align-items: center;
-  gap: 0.5rem;
-  padding: 0.5rem 1rem;
+  gap: 0.375rem;
+  padding: 0.375rem 0.75rem;        /* reduced padding */
   border-radius: var(--admin-radius-sm);
-  font-size: 0.75rem;
+  font-size: 0.6875rem;              /* reduced from 0.75rem */
   font-weight: 700;
   transition: all 150ms;
 }
@@ -216,11 +217,11 @@
 .admin-input {
   flex: 1;
   min-width: 12rem;
-  padding: 0.625rem 0.75rem;
+  padding: 0.5rem 0.625rem;         /* reduced padding */
   background-color: var(--admin-card-bg);
   border: 1px solid var(--admin-border);
   border-radius: var(--admin-radius-sm);
-  font-size: 0.875rem;
+  font-size: 0.8125rem;             /* 13px - reduced */
   color: var(--admin-text-primary);
   outline: none;
   transition: border-color 150ms;
@@ -245,9 +246,9 @@
 }
 
 .admin-filter-tab {
-  padding: 0.5rem 0.75rem;
+  padding: 0.375rem 0.625rem;       /* reduced padding */
   border-radius: var(--admin-radius-sm);
-  font-size: 0.75rem;
+  font-size: 0.6875rem;              /* reduced from 0.75rem */
   font-weight: 600;
   color: var(--admin-text-secondary);
   transition: all 150ms;