feat(ui): light-mode sky palette for Caddy card, favicon, doc sync

- index.css: add :root.light overrides for the sky-* accent used only by the
  Caddy settings card (buttons, badges, hovers) + the missing red-950/30 hover
- favicon: add public/favicon.svg (GhostGrid logo) and link it in index.html
- ARCHITECTURE.md: GET /caddy/routes returns a plain array, document the Caddy
  startup import, https:// upstream, favicon/public dir, and the SPA-catch-all-last
  + Cache-Control: no-store invariant

public/favicon.svg

@@ -0,0 +1,24 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 100 100">
+  <rect width="100" height="100" rx="22" fill="#0b0f19"/>
+  <!-- ghost body -->
+  <path d="M 24,78 C 18,65 14,35 34,22 C 48,12 62,15 68,26" stroke="#06b6d4" stroke-width="4" stroke-linecap="round" fill="none"/>
+  <path d="M 24,78 C 26,83 31,81 35,74 C 38,68 41,74 45,77 C 48,79 50,70 52,65" stroke="#06b6d4" stroke-width="4" stroke-linecap="round" fill="none"/>
+  <!-- eyes -->
+  <rect x="38" y="32" width="6" height="13" rx="3" fill="#00f0ff"/>
+  <rect x="52" y="32" width="6" height="13" rx="3" fill="#00f0ff"/>
+  <!-- network grid -->
+  <line x1="56" y1="38" x2="88" y2="38" stroke="#06b6d4" stroke-width="2.5"/>
+  <line x1="46" y1="62" x2="84" y2="62" stroke="#06b6d4" stroke-width="2.5"/>
+  <line x1="56" y1="20" x2="56" y2="80" stroke="#06b6d4" stroke-width="2.5"/>
+  <line x1="68" y1="15" x2="68" y2="76" stroke="#0891b2" stroke-width="2"/>
+  <line x1="80" y1="26" x2="80" y2="62" stroke="#06b6d4" stroke-width="2"/>
+  <!-- nodes -->
+  <circle cx="56" cy="26" r="4" fill="#00f0ff"/>
+  <circle cx="68" cy="26" r="4" fill="#00f0ff"/>
+  <circle cx="80" cy="26" r="4" fill="#00f0ff"/>
+  <circle cx="56" cy="38" r="4" fill="#00f0ff"/>
+  <circle cx="80" cy="38" r="5" fill="#38bdf8"/>
+  <circle cx="88" cy="38" r="4" fill="#00f0ff"/>
+  <circle cx="68" cy="62" r="5" fill="#38bdf8"/>
+  <circle cx="80" cy="62" r="4" fill="#00f0ff"/>
+</svg>