feat(builder): implement professional brand management experience

New BrandOverviewComponent, mounted at the top of the existing
Branding section page (no route/schema/business-logic changes):

- Completion ring + recommended next step + last-modified timestamp,
  computed from real facade signals (branding.logoUrl/faviconUrl/
  socialImageUrl, theme.palette, theme.typography, lastSavedAt) -
  nothing fabricated.
- Brand Health checklist (6 items: logo, favicon, colors, typography,
  social image, accessibility) - icon+text, never color alone.
- Color palette preview: 10 swatches from the real ThemePaletteConfig
  (primary/secondary/accent/success/warning/danger/backgrounds/text/
  border), each with a readable label.
- Real WCAG contrast check (contrast.util.ts, relative-luminance
  formula) between configured text and background colors - shows an
  inline warning when below 4.5:1, never blocks publishing.
- Typography live preview using the configured heading/body font
  families and base size.
- Social share preview card (Open Graph-style) using the existing
  socialImageUrl + seo.default.title/description, with an explicit
  empty state when no image is set yet.

Scope cut from the full brief given this session's cost already well
over budget entering this sprint - see Known limitations in the final
report (no multi-variant logo/favicon management, no live responsive
device preview, no new media library/asset picker; all reuse the
existing single-logo/single-favicon fields and image-field upload
component as-is).

Could not verify live in-browser this sprint: port 4200 is held by
another chat's dev server. Verified via `tsc --noEmit` (clean) and
manual template/binding review only; one binding (`--pct` custom-
property style binding) was replaced with a plain computed string to
remove an unverifiable risk rather than ship it unverified.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

src/app/i18n/translations.ts

@@ -760,6 +760,24 @@ export interface Translations {
     groupPreviewPurpose: string;
     groupPreviewWhere: string;
     groupPreviewSkip: string;
+    brandOverviewTitle: string;
+    brandOverviewSubtitle: string;
+    brandOverviewComplete: string;
+    brandLastModified: string;
+    brandFaviconUploaded: string;
+    brandColorsSelected: string;
+    brandTypographyConfigured: string;
+    brandSocialImageUploaded: string;
+    brandAccessibilityPassed: string;
+    brandColorsPreviewTitle: string;
+    brandTypographyPreviewTitle: string;
+    brandSocialPreviewTitle: string;
+    brandTypeSampleHeading: string;
+    brandTypeSampleBody: string;
+    brandContrastWarning: string;
+    brandSocialEmpty: string;
+    brandAccentColor: string;
+    brandBorderColor: string;
   };
   dashboard: {
     title: string;