feat: build Global Settings and Product Editor (F2.18-F2.19)

Settings:
- GlobalSettings page with tabbed interface
- Replace WPCFTO framework with React components
- Tabs: General, Payment, Pages, Customer
- Multicurrency settings with default currency selector
- AJAX save functionality with status feedback

Product Editor:
- VariationPricingTable - Complete recreation of Vue app
- Multi-currency flat pricing (columns mode)
- Multi-currency expanded mode (inner tables per variation)
- Dynamic rows from attribute repeater (MutationObserver + polling)
- Decimal digits per currency (affects step value)
- Required field validation (default currency price required)
- Real-time JSON update to hidden input
- SweetAlert2 integration for validation errors
- Stock and weight fields per variation
- Delete variation support

Migration:
- Preserves data compatibility with Vue app format
- Same data structure: {key, name, stock, weight, active, prices[]}
- Prices array with currency triple format: 'code:::name:::symbol'
- Sorts default currency first in prices array

See MIGRATION_STRATEGY.md for full migration details

src/admin/components/settings/GlobalSettings.css

@@ -0,0 +1,110 @@
+.formipay-global-settings {
+    display: flex;
+    flex-direction: column;
+    height: 100%;
+    background: #f6f7f7;
+}
+
+.formipay-settings-header {
+    display: flex;
+    justify-content: space-between;
+    align-items: center;
+    padding: 16px 20px;
+    background: #fff;
+    border-bottom: 1px solid #e0e0e0;
+}
+
+.formipay-settings-header h1 {
+    margin: 0;
+    font-size: 20px;
+    font-weight: 600;
+    display: flex;
+    align-items: center;
+    gap: 10px;
+}
+
+.formipay-settings-header svg {
+    fill: #1e1e1e;
+}
+
+.header-actions {
+    display: flex;
+    align-items: center;
+    gap: 12px;
+}
+
+.save-success {
+    color: #28a745;
+    font-size: 13px;
+    font-weight: 600;
+}
+
+.save-error {
+    color: #dc3545;
+    font-size: 13px;
+    font-weight: 600;
+}
+
+.formipay-settings-content {
+    display: flex;
+    flex: 1;
+    overflow: hidden;
+}
+
+.formipay-settings-tabs {
+    display: flex;
+    flex-direction: column;
+    width: 200px;
+    background: #fff;
+    border-right: 1px solid #e0e0e0;
+}
+
+.tab-button {
+    display: flex;
+    align-items: center;
+    gap: 10px;
+    padding: 12px 16px;
+    border: none;
+    background: transparent;
+    text-align: left;
+    cursor: pointer;
+    transition: background 0.2s;
+    border-left: 3px solid transparent;
+}
+
+.tab-button:hover {
+    background: #f6f7f7;
+}
+
+.tab-button.is-active {
+    background: #f0f6fc;
+    border-left-color: #2271b1;
+}
+
+.tab-button .dashicons {
+    font-size: 16px;
+    width: 16px;
+    height: 16px;
+}
+
+.formipay-settings-panel {
+    flex: 1;
+    padding: 20px;
+    overflow-y: auto;
+}
+
+.settings-panel {
+    max-width: 800px;
+}
+
+.settings-panel .components-panel__body {
+    border: 1px solid #e0e0e0;
+}
+
+.form-label {
+    display: block;
+    font-size: 13px;
+    font-weight: 600;
+    color: #1e1e1e;
+    margin-bottom: 4px;
+}