feat: update branding with official HiHala logo + 'Museums Data'

- Add official HiHala logo SVG (text portion from v1.5)
- Replace text-based nav brand with logo image + 'Museums Data'
- Update data source to default to NocoDB with Sheets fallback
- Move all credentials to environment variables (.env.local)

.env.example

@@ -1,3 +1,7 @@
-# NocoDB Configuration (optional - only needed if using NocoDB as data source)
+# NocoDB (primary data source)
 REACT_APP_NOCODB_URL=http://localhost:8090
 REACT_APP_NOCODB_TOKEN=your_token_here
+
+# Google Sheets (fallback if NocoDB fails)
+REACT_APP_SHEETS_ID=your_spreadsheet_id_here
+REACT_APP_SHEETS_NAME=Consolidated Data