Refactor routing and documentation for SPA deployment

- Updated the application to serve the SPA from the root path instead of /map/, enhancing accessibility.
- Modified redirect logic to ensure backward compatibility with old /map/* URLs.
- Adjusted documentation across multiple files to reflect the new routing structure and API endpoints.
- Improved handling of OAuth redirects and session management in the backend.
- Updated frontend configuration to align with the new base URL settings.

CONTRIBUTING.md

@@ -10,7 +10,7 @@ Clone the repository and run the project locally (see [Development](docs/develop
 ## Code layout
 
 - **Backend:** Entry point is `cmd/hnh-map/main.go`. All application logic lives in `internal/app/` (package `app`): `app.go`, `auth.go`, `api.go`, `handlers_redirects.go`, `client.go`, `client_grid.go`, `client_positions.go`, `client_markers.go`, `admin_*.go`, `map.go`, `tile.go`, `topic.go`, `migrations.go`.
-- **Frontend:** Nuxt 3 app in `frontend-nuxt/` (pages, components, composables, layouts, server, plugins, `public/gfx`). It is served by the Go backend under `/map/` with baseURL `/map/`.
+- **Frontend:** Nuxt 3 app in `frontend-nuxt/` (pages, components, composables, layouts, server, plugins, `public/gfx`). It is served by the Go backend at root `/` with baseURL `/`.
 
 ## Formatting and tests