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/lib/components/BottomNav.svelte

@@ -39,7 +39,7 @@
       color: var(--muted); font-size: 0.7rem; letter-spacing: 0.02em;
     }
     .bottomnav button svg { width: 23px; height: 23px; }
-    .bottomnav button.active { color: var(--sage-deep); }
-    .bottomnav button.active svg { stroke: var(--sage); }
+    .bottomnav button.active { color: var(--accent-deep); }
+    .bottomnav button.active svg { stroke: var(--accent); }
   }
 </style>