feat: add base_path support for mounting under a URL prefix

Wraps the router in axum's `nest` when BOOK_BASE_PATH is set, prepends
the prefix to form actions and asset URLs, and exposes a {{base}}
template placeholder. basePath option added to the NixOS module.

templates/default.html

@@ -15,6 +15,8 @@
     entries - Approved guestbook entries, newest first.
     style   - Custom CSS from BOOK_STYLE or BOOK_STYLE_FILE, wrapped in
               a <style> tag. Uses built-in default.css when neither is set.
+    base    - URL prefix the guestbook is mounted at (BOOK_BASE_PATH).
+              Empty when serving at the domain root.
 
   See default.css for available CSS classes on rendered elements.
 -->