feat(backend): password login, session gate, and list page

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

backend/templates/login.html

@@ -0,0 +1,24 @@
+{{define "login"}}
+<!doctype html>
+<html lang="en">
+<head>
+  <meta charset="utf-8">
+  <meta name="viewport" content="width=device-width, initial-scale=1, viewport-fit=cover">
+  <meta name="color-scheme" content="dark light">
+  <title>mangaBookmark</title>
+  <link rel="stylesheet" href="/static/style.css">
+</head>
+<body class="login-body">
+  <main class="login-card">
+    <h1>mangaBookmark</h1>
+    <form method="post" action="/login">
+      <label for="password">Password</label>
+      <input id="password" name="password" type="password"
+             autocomplete="current-password" autofocus required>
+      {{if .Error}}<p class="error">{{.Error}}</p>{{end}}
+      <button type="submit">Sign in</button>
+    </form>
+  </main>
+</body>
+</html>
+{{end}}