refactor(config): rewrite config parser

This PR refactors the configuration parser, replacing the old parser implementation with a new, more structured approach that includes validation and improved organization.

Key changes:
- Complete rewrite of the configuration parser using a map-based structure with built-in validation
- Addition of comprehensive validator functions for configuration values
- Renamed numerous configuration getter methods for better consistency

internal/ui/oauth2_callback.go

@@ -145,7 +145,7 @@ func (h *handler) oauth2Callback(w http.ResponseWriter, r *http.Request) {
 	http.SetCookie(w, cookie.New(
 		cookie.CookieUserSessionID,
 		sessionToken,
-		config.Opts.HTTPS,
+		config.Opts.HTTPS(),
 		config.Opts.BasePath(),
 	))