feat: add searchable units list with filtering

lib/ai/unit_list.dart

@@ -0,0 +1,82 @@
+import 'package:flutter/material.dart';
+import '../corpus.dart';
+import '../formula_models.dart';
+
+class UnitList extends StatefulWidget {
+  final UnitCorpus corpus;
+
+  const UnitList({super.key, required this.corpus});
+
+  @override
+  State<UnitList> createState() => _UnitListState();
+}
+
+class _UnitListState extends State<UnitList> {
+  final TextEditingController _searchController = TextEditingController();
+  String _searchQuery = '';
+
+  @override
+  void initState() {
+    super.initState();
+    _searchController.addListener(_onSearchChanged);
+  }
+
+  void _onSearchChanged() {
+    setState(() {
+      _searchQuery = _searchController.text.toLowerCase();
+    });
+  }
+
+  @override
+  void dispose() {
+    _searchController.dispose();
+    super.dispose();
+  }
+
+  List<UnitSpec> get _filteredUnits {
+    bool filter(UnitSpec unit) => unit.name.toLowerCase().contains(_searchQuery);
+    return widget.corpus.allUnits().where(filter).toList();
+  }
+
+  // Add unit conversion to base unit to the unit widget AI!
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: AppBar(
+        title: const Text('Units List'),
+      ),
+      body: Column(
+        children: [
+          Padding(
+            padding: const EdgeInsets.all(16.0),
+            child: TextField(
+              controller: _searchController,
+              decoration: const InputDecoration(
+                labelText: 'Search units',
+                hintText: 'Start typing unit name...',
+                prefixIcon: Icon(Icons.search),
+                border: OutlineInputBorder(),
+              ),
+            ),
+          ),
+          Expanded(
+            child: ListView.builder(
+              itemCount: _filteredUnits.length,
+              itemBuilder: (context, index) {
+                final unit = _filteredUnits[index];
+                return ListTile(
+                  title: Text(unit.name),
+                  subtitle: Text('Symbol: ${unit.symbol} • Base unit: ${unit.baseUnit}'),
+                  contentPadding: const EdgeInsets.symmetric(horizontal: 16, vertical: 8),
+                  tileColor: index.isEven
+                      ? Theme.of(context).colorScheme.surfaceVariant.withOpacity(0.3)
+                      : null,
+                );
+              },
+            ),
+          ),
+        ],
+      ),
+    );
+  }
+}