Add server-side subpath support for WebUI (#1658)

* Add server-side subpath support for WebUI

- Add webUISubpath configuration setting with regex validation
- Create temporary WebUI directory for subpath serving
- Inject subpath config into index.html for client detection
- Support all WebUI flavors (WEBUI, VUI, CUSTOM) with subpath
- Update browser opening to use subpath URLs

Related to Suwayomi/Suwayomi-WebUI#174

* Fix review points

* Fix code formatting issues

* Fix import issue

server/server-config/src/main/kotlin/suwayomi/tachidesk/server/ServerConfig.kt

@@ -759,6 +759,16 @@ class ServerConfig(
         description = "Strategy to apply when remote progress is older than local.",
     )
 
+    val webUISubpath: MutableStateFlow<String> by StringSetting(
+        protoNumber = 75,
+        group = SettingGroup.WEB_UI,
+        defaultValue = "",
+        pattern = "^(/[a-zA-Z0-9._-]+)*$".toRegex(),
+        description = "Serve WebUI under a subpath (e.g., /manga). Leave empty for root path. Must start with / if specified.",
+        requiresRestart = true,
+    )
+
+
     /** ****************************************************************** **/
     /**                                                                    **/
     /**                          Renamed settings                          **/