feat(theme): VNClagoon brand theme (navy + cyan) as default, dark-first

Add builtin-vnclagoon theme (cyan #00D4FF accent on navy #0A0E1A, DM Sans body
+ Syne headings, self-hosted OFL fonts); set as default theme policy; default
mode dark. Add VNCmail wordmark SVGs (on-dark/on-light) + wire logo/company via
k8s secret template. Placeholder wordmark — swap official styleguide SVG.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/branding/vncmail-wordmark-on-dark.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 560 130" role="img" aria-label="VNCmail">
+  <!-- Placeholder wordmark — VNClagoon palette. Swap for the official styleguide SVG when available. -->
+  <text x="280" y="98" text-anchor="middle"
+        font-family="'Avenir Next','Futura','Montserrat','Helvetica Neue',Arial,sans-serif"
+        font-weight="800" font-size="96" letter-spacing="-2">
+    <tspan fill="#EEF2FC">VNC</tspan><tspan fill="#00D4FF">mail</tspan>
+  </text>
+</svg>

public/branding/vncmail-wordmark-on-light.svg

@@ -0,0 +1,8 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 560 130" role="img" aria-label="VNCmail">
+  <!-- Placeholder wordmark — VNClagoon palette (light backgrounds). Swap for the official styleguide SVG when available. -->
+  <text x="280" y="98" text-anchor="middle"
+        font-family="'Avenir Next','Futura','Montserrat','Helvetica Neue',Arial,sans-serif"
+        font-weight="800" font-size="96" letter-spacing="-2">
+    <tspan fill="#0A0E1A">VNC</tspan><tspan fill="#00B4E0">mail</tspan>
+  </text>
+</svg>