feat(ui) : add site colors, dark mode toggle and card styling improvements

- Site color field with color picker in create/edit modals
- Dark mode theme (mytheme-dark) with toggle in navbar
- Stronger site color visibility on cards (gradient, top border, badges)
- Bigger action buttons (btn-sm) on machine cards
- White card backgrounds with proper dark mode support

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/components/sites/SiteCard.vue

@@ -1,11 +1,21 @@
 <template>
-  <div class="card bg-base-100 shadow-lg hover:shadow-xl transition-shadow">
+  <div
+    class="card site-card shadow-md hover:shadow-xl transition-shadow overflow-hidden"
+    :style="{
+      borderTop: site.color ? `4px solid ${site.color}` : '4px solid transparent',
+      background: site.color ? `linear-gradient(160deg, ${site.color}30 0%, ${site.color}08 40%, var(--color-base-100) 100%)` : undefined,
+    }"
+  >
     <div class="card-body">
       <div class="flex items-center justify-between mb-4">
-        <h3 class="card-title text-lg">
+        <h3 class="card-title text-lg text-base-content">
           {{ site.name }}
         </h3>
-        <div class="badge badge-primary badge-sm">
+        <div
+          class="badge font-bold"
+          :style="site.color ? { backgroundColor: site.color + '30', color: site.color, borderColor: site.color + '50' } : {}"
+          :class="!site.color ? 'badge-primary' : ''"
+        >
           {{ machineCount }} machines
         </div>
       </div>