feat: build React Form Builder (F2.7-F2.10)

Components:
- FieldPalette: drag-and-drop source for all field types
- FormCanvas: drop target with field list, reordering, CRUD
- FormField: individual field component with actions
- FieldSettingsPanel: edit field properties (label, ID, options, etc.)
- FormFieldOptions: manage select/radio/checkbox options
- FormPreview: live preview of rendered form
- FormFieldPreview: preview individual field types

Features:
- 16 field types (text, email, select, checkbox, radio, etc.)
- Categorized field palette
- Drag-and-drop field reordering
- Per-field settings panel
- Option management for choice fields
- Live form preview
- Save via AJAX

Config:
- fieldTypes.js: field definitions and constants
- Generate unique field IDs
- Field type categories (input, choice, layout, preset, advanced)

src/admin/components/formBuilder/FormFieldPreview.css

@@ -0,0 +1,87 @@
+.formipay-preview-form {
+    display: flex;
+    flex-direction: column;
+    gap: 16px;
+}
+
+.formipay-field-preview {
+    display: flex;
+    flex-direction: column;
+    gap: 6px;
+}
+
+.formipay-field-label {
+    display: block;
+    font-size: 13px;
+    font-weight: 500;
+    color: #1e1e1e;
+}
+
+.formipay-field-label .required {
+    color: #d63638;
+    margin-left: 2px;
+}
+
+.formipay-input,
+.formipay-textarea,
+.formipay-select {
+    width: 100%;
+    padding: 8px 12px;
+    font-size: 13px;
+    color: #1e1e1e;
+    background: #fff;
+    border: 1px solid #8c8f94;
+    border-radius: 2px;
+}
+
+.formipay-input:focus,
+.formipay-textarea:focus,
+.formipay-select:focus {
+    outline: none;
+    border-color: #2271b1;
+    box-shadow: 0 0 0 1px #2271b1;
+}
+
+.formipay-textarea {
+    resize: vertical;
+    min-height: 80px;
+}
+
+.formipay-radio-group,
+.formipay-checkbox-group {
+    display: flex;
+    flex-direction: column;
+    gap: 8px;
+}
+
+.formipay-radio-label,
+.formipay-checkbox-label {
+    display: flex;
+    align-items: center;
+    gap: 8px;
+    font-size: 13px;
+    color: #1e1e1e;
+    cursor: not-allowed;
+}
+
+.formipay-field-description {
+    margin: 0;
+    font-size: 12px;
+    color: #646970;
+}
+
+.formipay-divider {
+    border: none;
+    border-top: 1px solid #c3c4c7;
+    margin: 8px 0;
+}
+
+.formipay-page-break {
+    padding: 12px;
+    text-align: center;
+    background: #f6f7f7;
+    border: 1px dashed #c3c4c7;
+    border-radius: 2px;
+    font-size: 12px;
+    color: #646970;
+}