chore: name the actions column, drop hardcoded tunnel port and 2 KB mentions

The tunnel port default lives only in tunnel.DefaultPort; docs and
comments refer to "the tunnel port". The file manager hint no longer
states an edit limit that was not the real one.

internal/server/web/templates/dashboard.html

@@ -22,7 +22,7 @@
   <div class="card">
     {{if .Agents}}
     <table>
-      <thead><tr><th>Статус</th><th>Имя</th><th>Hostname</th><th>ОС / Арх</th><th>Активность</th><th></th></tr></thead>
+      <thead><tr><th>Статус</th><th>Имя</th><th>Hostname</th><th>ОС / Арх</th><th>Активность</th><th>Действия</th></tr></thead>
       <tbody>
       {{range .Agents}}
         <tr data-agent="{{.ID}}">