feat(f1): per-tool settings memory (color/width per tool) pure model

ToolSettings: the active tool + each tool's OWN remembered ToolConfig
(color/width), so pen→highlighter→pen restores the pen's last color/width
instead of bleeding the highlighter's. Immutable copy-on-write (withActive/
withColor/withWidth only touch the active tool); sensible defaults (black thin
pen, yellow fat highlighter, medium eraser). The toolbar holds + persists one.

Pure model; fully unit-tested (per-tool isolation, immutability, equality).

flutter analyze lib/editor clean; 204/204 tests (+6).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

test/tool_settings_test.dart

@@ -0,0 +1,58 @@
+// Tests for per-tool settings memory (F1/F5/F11).
+
+import 'package:flutter_test/flutter_test.dart';
+
+import 'package:badnote/editor/input/tool_settings.dart';
+
+void main() {
+  test('defaults: pen active, per-tool colors/widths', () {
+    final s = ToolSettings.defaults();
+    expect(s.active, EditorToolType.pen);
+    expect(s.configFor(EditorToolType.pen).color, 0xFF000000);
+    expect(s.configFor(EditorToolType.highlighter).width, 0.02);
+    expect(s.activeConfig, s.configFor(EditorToolType.pen));
+  });
+
+  test('switching tools remembers each tool\'s own config', () {
+    var s = ToolSettings.defaults();
+    // Customize pen, switch to highlighter, customize it, switch back.
+    s = s.withColor(0xFFFF0000).withWidth(0.005); // pen → red, 0.005
+    s = s.withActive(EditorToolType.highlighter);
+    expect(s.activeConfig.color, 0x80FFEB3B); // highlighter unchanged
+    s = s.withColor(0x8000FF00); // highlighter → green
+    s = s.withActive(EditorToolType.pen);
+    // Pen restored to its red/0.005, NOT the highlighter's green.
+    expect(s.activeConfig.color, 0xFFFF0000);
+    expect(s.activeConfig.width, 0.005);
+    // Highlighter kept its green.
+    expect(s.configFor(EditorToolType.highlighter).color, 0x8000FF00);
+  });
+
+  test('withColor/withWidth only touch the active tool', () {
+    var s = ToolSettings.defaults().withActive(EditorToolType.highlighter);
+    final penBefore = s.configFor(EditorToolType.pen);
+    s = s.withColor(0x80123456);
+    expect(s.configFor(EditorToolType.pen), penBefore); // pen untouched
+    expect(s.activeConfig.color, 0x80123456);
+  });
+
+  test('immutability: updates return new instances, original unchanged', () {
+    final a = ToolSettings.defaults();
+    final b = a.withActive(EditorToolType.eraser);
+    expect(a.active, EditorToolType.pen);
+    expect(b.active, EditorToolType.eraser);
+    expect(a == b, isFalse);
+  });
+
+  test('value equality', () {
+    expect(ToolSettings.defaults(), ToolSettings.defaults());
+    expect(ToolSettings.defaults().withActive(EditorToolType.eraser) ==
+        ToolSettings.defaults(), isFalse);
+  });
+
+  test('ToolConfig copyWith + equality', () {
+    const c = ToolConfig(color: 0xFF000000, width: 0.003);
+    expect(c.copyWith(width: 0.01), const ToolConfig(color: 0xFF000000, width: 0.01));
+    expect(c.copyWith(), c);
+  });
+}