feat: UI revamp with sidebar nav and grouped library

Replace top Nav with Sidebar, add lib/group.js for library grouping,
tokenized color/alert styles, and tighter Settings/Canvas export layout.
Adds @tabler/icons-react. Verified production build (standalone) passes,
so the Docker image built from this tree matches the local app.

Co-authored-by: Claude <claude-code@anthropic.com>

lib/group.js

@@ -0,0 +1,22 @@
+// lib/group.js — group saved assignments by subject for the sidebar tree and
+// the Library page. Shared so both stay in sync.
+
+export function groupBySubject(items) {
+  const map = new Map();
+  for (const a of items || []) {
+    const key = (a.subject && a.subject.trim()) || a.gradeLevel || "Other";
+    if (!map.has(key)) map.set(key, []);
+    map.get(key).push(a);
+  }
+  const groups = [...map.entries()].map(([key, list]) => ({
+    key,
+    items: list.slice().sort((x, y) => String(y.updatedAt || "").localeCompare(String(x.updatedAt || ""))),
+  }));
+  // Stable, friendly order: alphabetical by subject, "Other" last.
+  groups.sort((a, b) => {
+    if (a.key === "Other") return 1;
+    if (b.key === "Other") return -1;
+    return a.key.localeCompare(b.key);
+  });
+  return groups;
+}