Add configurable BasePath for subfolder hosting and update frontend base handling

IIS.md

@@ -10,8 +10,9 @@
 - From repo root: `dotnet publish -c Release -o publish`
 - Copy `publish/` contents to site directory (keep `App_Data` writable by the app pool user).
 - Set environment variables in web.config or IIS config:
-  - `ASPNETCORE_ENVIRONMENT=Production`
-  - `ADMIN_PASSWORD=<your-secret>`
+- `ASPNETCORE_ENVIRONMENT=Production`
+- `ADMIN_PASSWORD=<your-secret>`
+- `BasePath=/vote` (only if the site is under a subfolder; omit for root)
 - Optional: enable stdout logging in `web.config` during troubleshooting only; disable afterward.
 
 ## Permissions