feat: add native subpath support via server.path_prefix

[ajinfajrian]

Context & Motivation

Currently, Vigil is hardcoded to run at the root of a domain (e.g., https://status.example.com/ ). Running it under a subpath (e.g., https://example.com/status ) is not fully supported because static assets, templates, manager API endpoints, and JavaScript requests assume they own the domain root.

This PR adds native subpath support via a new optional path_prefix configuration key under the [server] block.

Live Demonstration

A live demonstration of this feature in action can be viewed at:
https://ajinf.my.id/status (where all routes, stylesheets, fonts, logo links, and AJAX polling calls successfully resolve under the /status subpath directory).

Summary of Changes

1. Config Option: Added an optional path_prefix string parameter in the [server] config block.
2. Backend Route Scoping: If a path_prefix is configured, Actix-Web routes and MCP services are mounted under a web::scope(&prefix) . The trimmed scope root path ( "" ) is also mapped to the main index handler to handle trailing slash normalizations correctly.
3. Template Context: Normalized the prefix and passed it to the Tera templates context ( config.path_prefix ).
4. Front-End Prefixing:
   • Prepended config.path_prefix to the logo, stylesheets, and javascript links in index.tera .
   • Exposed window.PATH_PREFIX to the frontend JavaScript.
   • Prepended window.PATH_PREFIX to the frontend AJAX polling endpoints in index.js .
5. Asset Path Resolution: Converted hardcoded root-relative URLs ( /assets/... ) inside common.css to relative paths ( ../... ) so fonts and badge icons resolve correctly under any custom subpath directory.
6. Docs: Added path_prefix documentation to config.cfg and README.md .

Backward Compatibility

• If path_prefix is unset or set to an empty string (the default), the behavior remains byte-for-byte identical to the current root-relative deployment setup.
• Existing deployments will not notice any change.