site-builder: feedback batch (social, features, header menu, spacer)

Five of six items from user feedback (Contact Form email delivery split
into a focused, live-tested follow-up):

- Social Links: add Spotify + Twitch (FA 4.7.0 already ships both glyphs).
- Features Grid: per-feature icon/image toggle (upload + URL) and an
  optional button (text + url); render, settings, and HTML export updated,
  backward compatible with existing icon-only features.
- Header: seed the default header with a Navbar (logo + Home/About/Services/
  Contact) so new sites open with an editable menu-with-links instead of an
  empty header zone. Adds a vitest guard that the seed deserializes and
  exports a real <nav>.
- Canvas: slim the empty header/footer placeholder from a padded band to a
  thin hint line so an empty zone no longer reads as a stray spacer.

Design spec: docs/superpowers/specs/2026-07-06-site-builder-feedback-batch-design.md

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

craft/src/components/basic/SocialLinks.tsx

@@ -29,6 +29,8 @@ const platformIcons: Record<string, string> = {
   pinterest: 'fa-pinterest',
   snapchat: 'fa-snapchat',
   whatsapp: 'fa-whatsapp',
+  spotify: 'fa-spotify',
+  twitch: 'fa-twitch',
 };
 
 const platformLabels: Record<string, string> = {
@@ -42,6 +44,8 @@ const platformLabels: Record<string, string> = {
   pinterest: 'Pinterest',
   snapchat: 'Snapchat',
   whatsapp: 'WhatsApp',
+  spotify: 'Spotify',
+  twitch: 'Twitch',
 };
 
 const allPlatforms = Object.keys(platformIcons);