Rebrand to the azure logo + warm sand/sea/sun palette

- Add the real Upbeat Bytes lockup (logo.svg) and use it in the header,
  replacing the placeholder inline mark + text wordmark.
- New square favicon: the logo's rising sun (bright gold) on azure.
- Recolor the design system around the logo's #0083ad azure: rename the
  --sage* accent vars to --accent*, with deep/soft azure tints; navy ink
  (#16263a) echoing the logo's "Bytes"; cool slate muted text; a deep gold
  for text-weight accents plus --gold-bright for decorative fills; warm
  sand paper background. No urgency colors.
- Retint the hero image overlay and the no-image card gradients to match.
- theme-color → azure.

Built clean; frontend tests/build pass.

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

frontend/src/routes/+layout.svelte

@@ -19,7 +19,7 @@
     padding: 26px 0 34px;
     border-top: 1px solid var(--line);
   }
-  footer.site a { color: var(--sage-deep); }
+  footer.site a { color: var(--accent-deep); }
   /* room for the mobile bottom tab bar */
   @media (max-width: 720px) {
     footer.site { padding-bottom: calc(34px + 64px + env(safe-area-inset-bottom)); }