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/section.shared.scss

@@ -39,7 +39,8 @@ label {
   font-weight: 600;
 }
 
-label.full {
+label.full,
+app-form-field.full {
   grid-column: 1 / -1;
 }