fix: pass TOML proxy group extra string values without quotes

toml::format() serializes string values with surrounding quotes, so
`extra = { url = "http://..." }` produced `url: '"http://..."'` in the
Clash output. Use the raw string for string values and keep
toml::format() for other types.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/config/binding.h

@@ -88,7 +88,7 @@ namespace toml
             {
                 const auto& extra_table = toml::find<toml::table>(v, "extra");
                 for(const auto& [k, val] : extra_table)
-                    conf.Extras[k] = toml::format(val);
+                    conf.Extras[k] = val.is_string() ? val.as_string() : toml::format(val);
             }
             return conf;
         }