feat(tools): rnote-style toolbar core writing batch

Replace the ad-hoc tool palette with a shared tool system
(EditorToolKind) across the PDF, note and slide editors, and add
the core writing tools.

- Multiple brushes, each remembering its OWN color (rnote-style):
  selecting a brush restores its color, changing color updates only
  that brush, and each brush button shows its current color.
- Select tool: tap-select a committed stroke, drag to move it,
  delete it — persisted and undoable.
- Shape tool: line / rectangle / ellipse / arrow, drawn with a live
  preview and committed as generated PenStrokes (shape_geometry.dart)
  so they reuse stroke rendering, erase, persistence and undo.
- Highlighter + eraser fold into the same tool system.

Text/bookmark/search+OCR/backgrounds/Windows-Ink are later batches
(TODO). Brush opacity still deferred. analyze clean, 302 tests.

lib/l10n/app_zh.arb

@@ -42,6 +42,14 @@
   "brushBallpoint": "圆珠笔",
   "brushPencil": "铅笔",
   "brushHighlighter": "荧光笔",
+  "toolSelect": "选择",
+  "toolShape": "形状",
+  "shapePicker": "形状",
+  "shapeLine": "直线",
+  "shapeRectangle": "矩形",
+  "shapeEllipse": "椭圆",
+  "shapeArrow": "箭头",
+  "actionDeleteSelection": "删除所选",
   "actionUndo": "撤销",
   "actionRedo": "重做",
   "fingerDrawingOn": "手指书写：开",