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.css

@@ -1,16 +1,18 @@
-/* goodNews calm design system.
-   Warm but crisp: cream surfaces, sage accent, a serif voice for headlines,
-   strong readable contrast, generous space. No urgency colors (no red). */
+/* Upbeat Bytes calm design system.
+   Sand, sea, and sun: warm paper surfaces, a vivid-azure accent, gold highlight,
+   a serif voice for headlines, strong readable contrast, generous space.
+   No urgency colors (no red). Built around the logo's #0083ad azure. */
 :root {
-  --bg: #faf6ee;
+  --bg: #f7f4ec;
   --surface: #fffdf8;
-  --ink: #21281f;
-  --muted: #616c60;
-  --sage: #2f7d5b;
-  --sage-deep: #235e44;
-  --sage-soft: #e6efe6;
-  --line: #e7e1d4;
-  --gold: #a9802f;
+  --ink: #16263a;
+  --muted: #5d6b78;
+  --accent: #0083ad;
+  --accent-deep: #006b8e;
+  --accent-soft: #e0eef3;
+  --line: #e8e3d8;
+  --gold: #b3892b;
+  --gold-bright: #e6cd42;
 
   --serif: "Iowan Old Style", "Palatino Linotype", Palatino, Georgia, "Times New Roman", serif;
   --sans: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
@@ -42,7 +44,7 @@ h1, h2, h3 {
 a { color: inherit; text-decoration: none; }
 img { max-width: 100%; display: block; }
 button { font-family: inherit; cursor: pointer; }
-::selection { background: var(--sage-soft); }
+::selection { background: var(--accent-soft); }
 
 .container { max-width: var(--maxw); margin: 0 auto; padding: 0 20px; }
 .muted { color: var(--muted); }