Fix base URL for subdirectory deployment on pages.civilytics.org

- Set vite.config.mjs base to '/crdc-demo/' so asset paths resolve correctly
  when served from a subdirectory (not root)
- Add favicon.svg in public/ folder
- Update index.html with correct absolute paths including /crdc-demo/ prefix

vite.config.mjs

@@ -1,8 +1,11 @@
 import { defineConfig } from 'vite'
 import react from '@vitejs/plugin-react-swc'
 
+// The app is served at /crdc-demo/ on pages.civilytics.org.
+// Set base to the subdirectory so asset paths resolve correctly.
 export default defineConfig({
   plugins: [react()],
   server: { port: 5173 },
   build: { outDir: 'dist', sourcemap: true },
+  base: '/crdc-demo/',  // Required for subdirectory deployment on git-pages
 })