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>

package-lock.json

@@ -8,6 +8,7 @@
       "name": "mr-drews-assignment-creator",
       "version": "1.0.0",
       "dependencies": {
+        "@tabler/icons-react": "^3.44.0",
         "@tailwindcss/postcss": "^4.3.1",
         "next": "14.2.18",
         "postcss": "^8.5.15",
@@ -242,6 +243,32 @@
         "tslib": "^2.4.0"
       }
     },
+    "node_modules/@tabler/icons": {
+      "version": "3.44.0",
+      "resolved": "https://registry.npmjs.org/@tabler/icons/-/icons-3.44.0.tgz",
+      "integrity": "sha512-Wn0AOZG9sg0L+bjfMqq4eNhC6pQjIrk94LvvWYNYkY8KH8wC3YILRzQlrnVJc4FUeMxH/AK97QsYCX35H3LndA==",
+      "license": "MIT",
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/codecalm"
+      }
+    },
+    "node_modules/@tabler/icons-react": {
+      "version": "3.44.0",
+      "resolved": "https://registry.npmjs.org/@tabler/icons-react/-/icons-react-3.44.0.tgz",
+      "integrity": "sha512-8+rvzBbVm/1Z3sG3x7GUNAaxIKxwgz8xaMhRs23nrCnMTKRFAhEC+82zAIFeAA0seXdrAGX5HFCkaLpGK2rVHg==",
+      "license": "MIT",
+      "dependencies": {
+        "@tabler/icons": "3.44.0"
+      },
+      "funding": {
+        "type": "github",
+        "url": "https://github.com/sponsors/codecalm"
+      },
+      "peerDependencies": {
+        "react": ">= 16"
+      }
+    },
     "node_modules/@tailwindcss/node": {
       "version": "4.3.1",
       "resolved": "https://registry.npmjs.org/@tailwindcss/node/-/node-4.3.1.tgz",