feat(project-editor): add field descriptions to General and Branding sections (partial)

Adds a short plain-language description under each field label (new
.field-desc style, section.shared.scss) so a non-developer admin
understands what each field affects, per i18n (en/ru/hy) convention.

Only General and Branding sections are wired up so far - the *Desc i18n
keys for the remaining sections (theme/header/footer/homepage/widgets/
features/languages/navigation) were prepared in translations.ts/en/ru/hy
but not yet wired into their templates. Follow-up work.

src/app/features/project-editor/sections/general-section.component.html

@@ -4,22 +4,27 @@
   <div class="editor-grid two">
     <label>
       <span>{{ 'builder.marketplaceName' | translate }}</span>
+      <small class="field-desc">{{ 'builder.marketplaceNameDesc' | translate }}</small>
       <input type="text" [ngModel]="bootstrap.branding.brandName" (ngModelChange)="updateMarketplaceName($event)" />
     </label>
     <label>
       <span>{{ 'builder.domain' | translate }}</span>
+      <small class="field-desc">{{ 'builder.domainDesc' | translate }}</small>
       <input type="text" [ngModel]="bootstrap.tenant.host" (ngModelChange)="updateDomain($event)" />
     </label>
     <label class="full">
       <span>{{ 'builder.descriptionLabel' | translate }}</span>
+      <small class="field-desc">{{ 'builder.descriptionFieldDesc' | translate }}</small>
       <textarea rows="3" [ngModel]="bootstrap.seo.default.description" (ngModelChange)="updateDescription($event)"></textarea>
     </label>
     <label>
       <span>{{ 'builder.defaultLanguage' | translate }}</span>
+      <small class="field-desc">{{ 'builder.defaultLanguageDesc' | translate }}</small>
       <input type="text" [ngModel]="bootstrap.localization.defaultLocale" (ngModelChange)="updateDefaultLanguage($event)" />
     </label>
     <label>
       <span>{{ 'builder.supportedLanguages' | translate }}</span>
+      <small class="field-desc">{{ 'builder.supportedLanguagesDesc' | translate }}</small>
       <input type="text" [ngModel]="languagesValue()" (ngModelChange)="updateSupportedLanguages($event)" />
     </label>
   </div>