fix: improve UI and login functionality

* update UI components and layout
* fix login form functionality
* pin Docker base image to python:3.11-slim-bookworm

src/static/webssh.css

@@ -45,6 +45,33 @@ body {
 
 .btn-add:hover { background: #059669; }
 
+.sidebar-actions {
+  display: flex;
+  gap: 5px;
+  margin-bottom: 14px;
+  flex-wrap: nowrap;
+}
+
+.btn-sidebar-action {
+  flex: 1;
+  min-width: 0;
+  padding: 7px 6px;
+  color: #e5e7eb;
+  font-size: 12px;
+  white-space: nowrap;
+}
+
+.btn-terminal-action { background: #2563eb; }
+.btn-terminal-action:hover,
+.btn-terminal-action.active { background: #1d4ed8; }
+
+.btn-sftp-action { background: #d97706; }
+.btn-sftp-action:hover,
+.btn-sftp-action.active { background: #b45309; }
+
+.btn-sidebar-action:not(.btn-terminal-action):not(.btn-sftp-action) { background: #7c3aed; }
+.btn-sidebar-action:not(.btn-terminal-action):not(.btn-sftp-action):hover { background: #6d28d9; }
+
 .connection-list {
   flex: 1;
   overflow-y: auto;