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/preview-section.component.html

@@ -1,9 +1,9 @@
 <section class="editor-section-card">
   <h2>{{ 'builder.preview' | translate }}</h2>
   <div class="editor-actions">
-    <button type="button" (click)="preview()">{{ 'builder.livePreview' | translate }}</button>
-    <button type="button" class="secondary" (click)="refreshExport()">{{ 'builder.exportBootstrap' | translate }}</button>
-    <button type="button" class="secondary" (click)="importDraft()">{{ 'builder.importBootstrap' | translate }}</button>
+    <app-button variant="primary" (click)="preview()">{{ 'builder.livePreview' | translate }}</app-button>
+    <app-button variant="secondary" (click)="refreshExport()">{{ 'builder.exportBootstrap' | translate }}</app-button>
+    <app-button variant="secondary" (click)="importDraft()">{{ 'builder.importBootstrap' | translate }}</app-button>
   </div>
   @if (importError()) {
     <p class="editor-error">{{ importError()! | translate }}</p>