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/app.html

@@ -5,7 +5,7 @@
     <link rel="icon" href="%sveltekit.assets%/favicon.svg" />
     <link rel="manifest" href="%sveltekit.assets%/manifest.webmanifest" />
     <meta name="viewport" content="width=device-width, initial-scale=1" />
-    <meta name="theme-color" content="#2f7d5b" />
+    <meta name="theme-color" content="#0083ad" />
     <meta name="description" content="Calm, constructive news worth your attention — and nothing that isn't." />
     <title>Upbeat Bytes — calm, constructive news</title>
     %sveltekit.head%