feat(contact-button): add floating contact widget admin management

Add CMS support for floating contact widget with Facebook/Zalo quick
actions. Includes mongoose schema, admin UI tab, image upload with
sharp resize presets, deferred form submission with draft persistence,
and upload middleware error handling.

models/home.js

@@ -11,6 +11,51 @@ const LinkSchema = new Schema(
   { _id: false },
 );
 
+const FloatingContactBrandSchema = new Schema(
+  {
+    imageSrc: { type: String, default: "" },
+    imageAlt: { type: String, default: "", maxlength: 60 },
+  },
+  { _id: false },
+);
+
+const FloatingContactTriggerSchema = new Schema(
+  {
+    imageSrc: { type: String, default: "" },
+    icon: { type: String, default: "fa-comments" },
+  },
+  { _id: false },
+);
+
+const FloatingContactActionSchema = new Schema(
+  {
+    id: { type: String, default: "" },
+    platform: { type: String, default: "" },
+    enabled: { type: Boolean, default: true },
+    label: { type: String, default: "", maxlength: 48 },
+    subtitle: { type: String, default: "", maxlength: 48 },
+    href: { type: String, default: "" },
+    iconImage: { type: String, default: "" },
+    iconType: { type: String, default: "iconClass" },
+    iconClass: { type: String, default: "" },
+    iconText: { type: String, default: "", maxlength: 12 },
+    order: { type: Number, default: 0 },
+  },
+  { _id: false },
+);
+
+const FloatingContactSchema = new Schema(
+  {
+    enabled: { type: Boolean, default: true },
+    position: { type: String, default: "bottom-right" },
+    panelTitle: { type: String, default: "", maxlength: 72 },
+    brand: { type: FloatingContactBrandSchema, default: () => ({}) },
+    trigger: { type: FloatingContactTriggerSchema, default: () => ({}) },
+    actions: { type: [FloatingContactActionSchema], default: [] },
+  },
+  { _id: false },
+);
+
 // Hero slide (for multiple hero items in slider)
 const HeroSlideSchema = new Schema(
   {
@@ -266,6 +311,7 @@ const HomeSchema = new Schema(
     achievements: { type: AchievementsSchema, default: () => ({}) },
     partners: { type: PartnersSchema, default: () => ({}) },
     blogPreview: { type: BlogPreviewSchema, default: () => ({}) },
+    floatingContact: { type: FloatingContactSchema, default: () => ({}) },
   },
   {
     timestamps: true,