feat: web support relative base path (#1001)

ui/flutter/web/index.html

@@ -1,21 +1,6 @@
 <!DOCTYPE html>
 <html>
 <head>
-  <!--
-    If you are serving your web app in a path other than the root, change the
-    href value below to reflect the base path you are serving from.
-
-    The path provided below has to start and end with a slash "/" in order for
-    it to work correctly.
-
-    For more details:
-    * https://developer.mozilla.org/en-US/docs/Web/HTML/Element/base
-
-    This is a placeholder for base href that will be replaced by the value of
-    the `--base-href` argument provided to `flutter build`.
-  -->
-  <base href="$FLUTTER_BASE_HREF">
-
   <meta charset="UTF-8">
   <meta content="IE=Edge" http-equiv="X-UA-Compatible">
   <meta name="description" content="A modern download manager.">