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>

tests/widget-system/ticket-schema-version.test.cjs

@@ -120,5 +120,6 @@ test('TicketStatusService seeds mirror the migrator and guard the default row',
   assert.match(service, /if \(self::is_closed\(\$from\)\) \{\s*\n\s*return false;/);
   // 颜色只接受 #rgb / #rrggbb。
   assert.match(service, /'#6b7280'/);
-  assert.match(service, /preg_match\('\/\^#\(\?:\[0-9a-f\]\{3\}\|\[0-9a-f\]\{6\}\)\$\/'/);
+  assert.match(service, /\\AKSRT\\Core\\Color::sanitize\(\$color\)/);
+  assert.doesNotMatch(service, /preg_match\('\/\^#/);
 });