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

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

backend/static/filter.js

@@ -0,0 +1,6 @@
+// Title search and tab-state handling land in Task 7.
+function setActiveTab(el) {
+  el.parentElement.querySelectorAll("[role=tab]").forEach(function (t) {
+    t.classList.toggle("active", t === el);
+  });
+}

backend/static/style.css

@@ -0,0 +1,2 @@
+/* Styling lands in Task 7. */
+:root { color-scheme: dark light; }