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_en.arb

@@ -51,6 +51,14 @@
   "brushBallpoint": "Ballpoint",
   "brushPencil": "Pencil",
   "brushHighlighter": "Highlighter",
+  "toolSelect": "Select",
+  "toolShape": "Shape",
+  "shapePicker": "Shape",
+  "shapeLine": "Line",
+  "shapeRectangle": "Rectangle",
+  "shapeEllipse": "Ellipse",
+  "shapeArrow": "Arrow",
+  "actionDeleteSelection": "Delete selection",
   "actionUndo": "Undo",
   "actionRedo": "Redo",
   "fingerDrawingOn": "Finger drawing ON",