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/SiteCreateModal.vue

@@ -17,6 +17,46 @@
           />
         </div>
 
+        <div class="form-control">
+          <label class="label">
+            <span class="label-text">Couleur</span>
+          </label>
+          <div v-if="siteRef.color" class="flex items-center gap-3">
+            <input
+              :value="siteRef.color"
+              type="color"
+              class="w-10 h-10 rounded cursor-pointer border border-base-300"
+              :disabled="disabled"
+              @input="(e: Event) => { siteRef.color = (e.target as HTMLInputElement).value }"
+            >
+            <input
+              v-model="siteRef.color"
+              type="text"
+              placeholder="#000000"
+              class="input input-bordered input-sm flex-1"
+              :disabled="disabled"
+              maxlength="7"
+            >
+            <button
+              type="button"
+              class="btn btn-ghost btn-xs"
+              :disabled="disabled"
+              @click="siteRef.color = ''"
+            >
+              Effacer
+            </button>
+          </div>
+          <button
+            v-else
+            type="button"
+            class="btn btn-outline btn-sm w-fit"
+            :disabled="disabled"
+            @click="siteRef.color = '#3b82f6'"
+          >
+            Choisir une couleur
+          </button>
+        </div>
+
         <SiteContactFormFields :form="siteRef" :disabled="disabled" />
 
         <div class="modal-action">
@@ -39,6 +79,7 @@ import SiteContactFormFields from '~/components/sites/SiteContactFormFields.vue'
 
 type SiteForm = {
   name: string
+  color: string
   contactName: string
   contactPhone: string
   contactAddress: string