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

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

backend/templates/card.html

@@ -0,0 +1,18 @@
+{{define "card"}}
+<article class="card" id="card-{{.Key}}" data-title="{{.Title}}">
+  <a class="cover" href="{{.ContinueURL}}" target="_blank" rel="noopener noreferrer">
+    {{if .Cover}}<img src="{{.Cover}}" alt="" loading="lazy">{{end}}
+  </a>
+  <div class="body">
+    <h3 class="title">{{.Title}}</h3>
+    <p class="meta">
+      <span class="site site-{{.Site}}">{{.Site}}</span>
+      <span class="chapter">Ch {{.LastChapter}}</span>
+      {{if .HasNewChapter}}<span class="new">NEW {{.LatestChapter}}</span>{{end}}
+    </p>
+    <div class="actions">
+      <a class="primary" href="{{.ContinueURL}}" target="_blank" rel="noopener noreferrer">Continue</a>
+    </div>
+  </div>
+</article>
+{{end}}