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/homepage-section.component.html

@@ -24,7 +24,7 @@
           <label>
             <span>{{ 'builder.columns' | translate }}</span>
             <small class="field-desc">{{ 'builder.sectionColumnsDesc' | translate }}</small>
-            <input type="number" [ngModel]="section.layout?.columns || 1" (ngModelChange)="updateLayout(section.id, 'columns', $event)" />
+            <app-input type="number" [ngModel]="section.layout?.columns || 1" (ngModelChange)="updateLayout(section.id, 'columns', $event)" />
           </label>
         </div>
       </div>