feat: auto-resume session on page refresh

On load, check sessionStorage for stored credentials (saved during
login). If present, auto-authenticate and skip the login form. Falls
back to showing login if the stored credentials fail.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

crates/ericrfb-frontend/src/main.ts

@@ -1,5 +1,29 @@
 import './style.css'
 import { showLogin } from './login'
+import { mountShell } from './shell'
 
 const app = document.getElementById('app')!
+
+// Try to resume a session from stored credentials
+const user = sessionStorage.getItem('blekin_user')
+const pass = sessionStorage.getItem('blekin_pass')
+
+if (user && pass) {
+  app.innerHTML = '<div class="login"><div class="login-form"><h1>Reconnecting...</h1></div></div>'
+  fetch('/api/login', {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify({ username: user, password: pass }),
+  })
+    .then(r => r.ok ? r.json() : Promise.reject())
+    .then(data => {
+      mountShell(app, { appletId: data.applet_id, port: data.port, boardName: data.board_name })
+    })
+    .catch(() => {
+      sessionStorage.removeItem('blekin_user')
+      sessionStorage.removeItem('blekin_pass')
       showLogin(app)
+    })
+} else {
+  showLogin(app)
+}