add fullscreen hover tip

web/kiri/index.html

@@ -50,7 +50,7 @@
             <button id="redo"><i class="fas fa-redo-alt"></i></button>
         </div>
         <div class="grow"></div>
-        <div id="app-xpnd">
+        <div id="app-xpnd" lkt="fullscreen">
             <i class="fa-solid fa-maximize"></i>
         </div>
         <div id="app-acct">

web/kiri/lang/en.js

@@ -15,6 +15,7 @@ self.kiri.lang['en-us'] = {
     enable:         "enable",
     export:         "export",
     files:          "files",
+    fullscreen:     "fullscreen",
     help:           "help",
     ghost:          "ghost",        // left render pop menu (wireframe)
     hide:           "hide",         // left render pop menu (invisible)