feat(project-editor): adopt Design System primitives across editor sections

Applied app-input/app-form-field/app-button to 9 of 11 Project Editor
sections: general, branding, theme, footer, homepage, widgets, languages,
navigation, preview. header and features sections were left unchanged -
they contain only checkboxes and selects, and no Checkbox/Select primitive
exists yet.

Theme section's 8 color pickers stay native <input type=color> (app-input's
type union doesn't include 'color') but are now wrapped in app-form-field
for consistent label/hint treatment. Added app-form-field.full grid-column
rule to section.shared.scss (shared by all 11 sections) alongside the
existing label.full rule, since the custom element doesn't match that
selector.

Production build green, arch:check passes.

src/app/features/project-editor/sections/theme-section.component.ts

@@ -3,6 +3,7 @@ import { FormsModule } from '@angular/forms';
 import { ProjectEditorFacade } from '../facade/project-editor.facade';
 import { TranslatePipe } from '../../../i18n/translate.pipe';
 import { PlatformLayoutType, ThemePaletteConfig } from '../../../shared/models/config';
+import { FormFieldComponent } from '../../../shared/ui/form-field/form-field.component';
 
 export interface ThemeModeOption {
   value: 'light' | 'dark' | 'system';
@@ -19,7 +20,7 @@ export interface SiteLayoutOption {
 @Component({
   selector: 'app-project-editor-theme-section',
   standalone: true,
-  imports: [FormsModule, TranslatePipe],
+  imports: [FormsModule, TranslatePipe, FormFieldComponent],
   templateUrl: './theme-section.component.html',
   styleUrls: ['./section.shared.scss'],
   changeDetection: ChangeDetectionStrategy.OnPush