feat: Page Editor UX Phase 2 - unsaved changes panel, property search, reset property, empty states

- ProjectEditorFacade.resetField(key): field-level revert-to-original, reusing getByPath + a new immutable setByPath (write-side counterpart, scalar/object dot-paths only).
- app-form-field gains showReset/resetLabel/(resetClicked) - a reusable per-field reset affordance, wired on branding logo/title and theme primary/background color.
- Save bar: unsaved-changes count is now clickable, expanding a field-level diff list (reuses facade.changeSummary(), already built for the Preview tab) with jump-to-section links.
- BuilderPropertySearchComponent: filters EditorSchemaService.all() by translated label/hint, jumps to the owning section - no new registry, reuses the existing schema.
- navigation-section: app-empty-state (existing component) added for empty header/footer link lists.
- Reset Section and Draft/Published badge were already implemented; not touched.
- New copy added as translation keys (en/ru/hy).

src/app/i18n/en.ts

@@ -880,6 +880,16 @@ export const en: Translations = {
     usedByLinks: 'Links',
     usedByBadges: 'Badges',
     usedByPageBackground: 'Page background',
+    resetField: 'Reset',
+    propertySearchLabel: 'Search settings',
+    propertySearchPlaceholder: 'Search settings…',
+    propertySearchResultsLabel: 'Search results',
+    propertySearchNoResults: 'No matching settings.',
+    unsavedChangesPanelTitle: 'Unsaved changes',
+    navigationHeaderEmptyTitle: 'No header links yet',
+    navigationHeaderEmptyDesc: 'Add a link so customers can navigate from the header.',
+    navigationFooterEmptyTitle: 'No footer links yet',
+    navigationFooterEmptyDesc: 'Add a link so customers can navigate from the footer.',
     addBlockHint: 'Add a block to your homepage:',
     dragToReorder: 'Drag to reorder',
     blockRemoveConfirm: 'Remove this block from the homepage?',