refactor: migrate wechat to user attributes and enhance users list

Migrate the hardcoded wechat field to the new extensible user
attributes system and improve the users management UI.

Migration:
- Add migration 019 to move wechat data to user_attribute_values
- Remove wechat field from User entity, DTOs, and API contracts
- Clean up wechat-related code from backend and frontend

UsersView enhancements:
- Add text labels to action buttons (Filter Settings, Column Settings,
  Attributes Config) for better UX
- Change status column to show colored dot + Chinese text instead of
  English text
- Add dynamic attribute columns support with batch loading
- Add column visibility settings with localStorage persistence
- Add filter settings modal for search and filter preferences
- Update i18n translations

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/i18n/locales/en.ts

@@ -434,9 +434,7 @@ export default {
     administrator: 'Administrator',
     user: 'User',
     username: 'Username',
-    wechat: 'WeChat ID',
     enterUsername: 'Enter username',
-    enterWechat: 'Enter WeChat ID',
     editProfile: 'Edit Profile',
     updateProfile: 'Update Profile',
     updating: 'Updating...',
@@ -565,12 +563,10 @@ export default {
       email: 'Email',
       password: 'Password',
       username: 'Username',
-      wechat: 'WeChat ID',
       notes: 'Notes',
       enterEmail: 'Enter email',
       enterPassword: 'Enter password',
       enterUsername: 'Enter username (optional)',
-      enterWechat: 'Enter WeChat ID (optional)',
       enterNotes: 'Enter notes (admin only)',
       notesHint: 'This note is only visible to administrators',
       enterNewPassword: 'Enter new password (optional)',
@@ -582,7 +578,6 @@ export default {
       columns: {
         user: 'User',
         username: 'Username',
-        wechat: 'WeChat ID',
         notes: 'Notes',
         role: 'Role',
         subscriptions: 'Subscriptions',
@@ -653,7 +648,67 @@ export default {
       failedToDeposit: 'Failed to deposit',
       failedToWithdraw: 'Failed to withdraw',
       useDepositWithdrawButtons: 'Please use deposit/withdraw buttons to adjust balance',
-      insufficientBalance: 'Insufficient balance, balance cannot be negative after withdrawal'
+      insufficientBalance: 'Insufficient balance, balance cannot be negative after withdrawal',
+      // Settings Dropdowns
+      filterSettings: 'Filter Settings',
+      columnSettings: 'Column Settings',
+      filterValue: 'Enter value',
+      // User Attributes
+      attributes: {
+        title: 'User Attributes',
+        description: 'Configure custom user attribute fields',
+        configButton: 'Attributes',
+        addAttribute: 'Add Attribute',
+        editAttribute: 'Edit Attribute',
+        deleteAttribute: 'Delete Attribute',
+        deleteConfirm: "Are you sure you want to delete attribute '{name}'? All user values for this attribute will be deleted.",
+        noAttributes: 'No custom attributes',
+        noAttributesHint: 'Click the button above to add custom attributes',
+        key: 'Attribute Key',
+        keyHint: 'For programmatic reference, only letters, numbers and underscores',
+        name: 'Display Name',
+        nameHint: 'Name shown in forms',
+        type: 'Attribute Type',
+        fieldDescription: 'Description',
+        fieldDescriptionHint: 'Description text for the attribute',
+        placeholder: 'Placeholder',
+        placeholderHint: 'Placeholder text for input field',
+        required: 'Required',
+        enabled: 'Enabled',
+        options: 'Options',
+        optionsHint: 'For select/multi-select types',
+        addOption: 'Add Option',
+        optionValue: 'Option Value',
+        optionLabel: 'Display Text',
+        validation: 'Validation Rules',
+        minLength: 'Min Length',
+        maxLength: 'Max Length',
+        min: 'Min Value',
+        max: 'Max Value',
+        pattern: 'Regex Pattern',
+        patternMessage: 'Validation Error Message',
+        types: {
+          text: 'Text',
+          textarea: 'Textarea',
+          number: 'Number',
+          email: 'Email',
+          url: 'URL',
+          date: 'Date',
+          select: 'Select',
+          multi_select: 'Multi-Select'
+        },
+        created: 'Attribute created successfully',
+        updated: 'Attribute updated successfully',
+        deleted: 'Attribute deleted successfully',
+        reordered: 'Attribute order updated successfully',
+        failedToLoad: 'Failed to load attributes',
+        failedToCreate: 'Failed to create attribute',
+        failedToUpdate: 'Failed to update attribute',
+        failedToDelete: 'Failed to delete attribute',
+        failedToReorder: 'Failed to update order',
+        keyExists: 'Attribute key already exists',
+        dragToReorder: 'Drag to reorder'
+      }
     },
 
     // Groups