fix: enhance layout consistency in PluginInstall.page and default-base.css

- Added utility classes for width, height, and overflow to improve layout structure in PluginInstall.page.
- Removed the obsolete Plugins.css file as its styles are no longer needed.
- This change continues the effort to enhance visual consistency across the plugin.

emhttp/plugins/dynamix.plugin.manager/PluginInstall.page

@@ -65,7 +65,7 @@ Tag="download"
 
 :plugin_install_help:
 
-<div>
+<div class="max-w-20 mx-auto">
   <p><strong>_(Select local plugin file)_</strong></p>
-  <div id="plugin_tree" class="textarea"></div>
+  <div id="plugin_tree" class="textarea h-50 overflow-y-scroll scrollbar-thin"></div>
 </div>

emhttp/plugins/dynamix.plugin.manager/sheets/Plugins.css

@@ -1,8 +0,0 @@
-#plugin_tree {
-    width: 33%;
-    height: 200px;
-    overflow-y: scroll;
-}
-table tbody td {
-    line-height: normal;
-}