feat(pen): extensible brush model (4 brushes)

Replace the 2-tool ink system with a data-driven, Krita-style
BrushProfile (lib/editor/engine/brush.dart). Adding a brush is a
const map entry, not render-path branching.

Four presets from the rnote/krita spec:
- fountain pen: quadratic (p^2) pressure, wide dynamic width
- ballpoint:    near-constant width (thinning 0.15)
- highlighter:  flat width, square caps
- pencil:       sqrt(p) pressure, moderate width

Pressure is pre-warped per brush via PressureCurve(gamma) before
perfect_freehand; geometry fields (thinning/streamline/smoothing/
caps) flow through the shared stroke recipe so the PDF overlay and
the note/slide PenCanvas both honor the brush. Brush kind is now
persisted on the stroke model. Picker added to all three toolbars.

Opacity/multiply and pencil grain are carried as data but not yet
composited (TODO brush-opacity / brush-texture); this increment is
width + pressure-curve differentiation. analyze clean, 283 tests.

lib/l10n/app_localizations.dart

@@ -320,6 +320,36 @@ abstract class AppLocalizations {
   /// **'Eraser'**
   String get toolEraser;
 
+  /// No description provided for @brushPicker.
+  ///
+  /// In en, this message translates to:
+  /// **'Brush'**
+  String get brushPicker;
+
+  /// No description provided for @brushFountainPen.
+  ///
+  /// In en, this message translates to:
+  /// **'Fountain pen'**
+  String get brushFountainPen;
+
+  /// No description provided for @brushBallpoint.
+  ///
+  /// In en, this message translates to:
+  /// **'Ballpoint'**
+  String get brushBallpoint;
+
+  /// No description provided for @brushPencil.
+  ///
+  /// In en, this message translates to:
+  /// **'Pencil'**
+  String get brushPencil;
+
+  /// No description provided for @brushHighlighter.
+  ///
+  /// In en, this message translates to:
+  /// **'Highlighter'**
+  String get brushHighlighter;
+
   /// No description provided for @actionUndo.
   ///
   /// In en, this message translates to: