feat: Page Editor UX Phase 1 - live preview, hover mapping, visual layout picker

- PreviewHighlightService + appHighlightSource directive: shared hover/focus bridge between editor fields and the schematic live preview.
- BuilderLivePreviewComponent: in-page schematic homepage render (header/hero/blocks/footer) reading the same bootstrap the sections mutate, highlighting the area matching the active field.
- VisualLayoutPickerComponent: card-based layout picker (ControlValueAccessor, same shape as app-select) replacing the raw layout <select> in homepage-section.
- app-form-field gains optional usedBy/usedByLabel inputs for the "Where is this used?" helper text, wired into aria-describedby.
- Wired homepage/branding/theme sections with highlight sources + usedBy hints; live preview panel shown in project-editor-page for those three sections.
- All new copy added as translation keys (en/ru/hy).

src/app/i18n/translations.ts

@@ -869,6 +869,18 @@ export interface Translations {
     homepageBlocksHint: string;
     addBlockHint: string;
     dragToReorder: string;
+    livePreviewTitle: string;
+    livePreviewHint: string;
+    livePreviewEmpty: string;
+    livePreviewButtonLabel: string;
+    usedByLabel: string;
+    usedByHeader: string;
+    usedByFooter: string;
+    usedBySeo: string;
+    usedByButtons: string;
+    usedByLinks: string;
+    usedByBadges: string;
+    usedByPageBackground: string;
     blockRemoveConfirm: string;
     blockHero: string;
     blockHeroDesc: string;