docs: add o.cash logo as site favicon and nav icon

Extract the O icon from o.cash/logo.svg, use as favicon and navbar
logo. Add default theme re-export to fix build.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: niconiconi

docs/.vitepress/config.ts

@@ -4,6 +4,10 @@ export default defineConfig({
   title: '@ocash/sdk',
   description: 'TypeScript ZKP SDK for privacy-preserving token operations',
 
+  head: [
+    ['link', { rel: 'icon', type: 'image/svg+xml', href: '/logo.svg' }],
+  ],
+
   locales: {
     root: {
       label: 'English',
@@ -118,6 +122,7 @@ export default defineConfig({
   },
 
   themeConfig: {
+    logo: '/logo.svg',
     search: {
       provider: 'local',
     },

docs/.vitepress/theme/index.ts

@@ -0,0 +1,3 @@
+import DefaultTheme from 'vitepress/theme';
+
+export default DefaultTheme;