refactor(color): unify color sanitizing and picking behind shared components

- Add AKSRT\Core\Color::sanitize($value, $allow_alpha) as the single PHP
  color sanitizer; replace sanitize_hex_color calls and per-class regexes
  (FloatButtons, FieldSchema, ThemeSearch, icon cards, TicketStatusService)
- SchemaProvider / ControllerBase sanitize `color` fields as colors;
  alpha kept only when the schema sets showAlpha
- Split SchemaProvider OAuth validation and option matching into sibling
  traits to stay under the 1000-line limit
- Add AksColorPicker.vue + utils/color.js mirroring the PHP rules; AksField,
  FloatBtnCommonFields and TicketStatusPanel use it
- Widget background color input now uses wpColorPicker
- Remove dead AksCompositeField dimensions/spacing/background branches and
  the unused Gutenberg ColorPicker var
- Add color-unification contract test; update harness requires and tests

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

assets/admin/src/components/floating/FloatBtnCommonFields.vue

@@ -10,22 +10,20 @@
       <div class="float-common-fields__colors">
         <div class="float-common-fields__color">
           <span>自定义图标颜色</span>
-          <n-color-picker
-            :value="modelValue.color?.color || null"
-            :show-alpha="true"
-            :modes="['hex', 'rgb']"
+          <AksColorPicker
+            :model-value="modelValue.color?.color || ''"
+            alpha
             clearable
-            @update:value="updateColor('color', $event)"
+            @update:model-value="updateColor('color', $event)"
           />
         </div>
         <div class="float-common-fields__color">
           <span>自定义背景颜色</span>
-          <n-color-picker
-            :value="modelValue.color?.bg || null"
-            :show-alpha="true"
-            :modes="['hex', 'rgb']"
+          <AksColorPicker
+            :model-value="modelValue.color?.bg || ''"
+            alpha
             clearable
-            @update:value="updateColor('bg', $event)"
+            @update:model-value="updateColor('bg', $event)"
           />
         </div>
       </div>
@@ -35,6 +33,8 @@
 </template>
 
 <script setup>
+import AksColorPicker from '@/components/form/AksColorPicker.vue'
+
 const props = defineProps({
   modelValue: { type: Object, required: true },
 })