feat(flowchart): visible selection toolbar + console-log diagnostics; v4.10.0

Even after the v4.9.9 inline-modal fix the user kept reporting 'no fix still'
because hidden right-click context menus and window.prompt calls remain
unreliable in Electron renderer contexts. This release ships a *visible*
floating selection toolbar inside the canvas panel so the primary
interactions are not hidden behind modals or context menus.

- New <div id="fc-selection-toolbar"> inside #canvas-host, hidden until a
  node or edge is selected. Renders:
    * 5 shape buttons (Process / Decision / Terminator / Subroutine /
      Document) when a node is selected, calling store.setNodeKind on
      click. The active shape is highlighted.
    * 3 edge-kind buttons (Solid / Dotted / Thick) when an edge is
      selected, calling store.setEdgeKind.
    * An always-visible label input that mirrors the selected item's
      label and writes back via store.setNodeLabel / store.setEdgeLabel
      with a 100ms debounce.
    * A red Delete button calling store.removeNode / store.disconnect.
- Selection state now tracked at the controller level (_selectedId +
  _selectedKind) and exposed via window.FlowchartController so the
  keyboard Delete/Backspace shortcut and the toolbar share one source
  of truth.
- Console-log instrumentation on every canvas event (pointerdown with
  altKey, pointerup with drag result, dblclick, contextmenu, selection
  changes, and every bootstrap phase) so the user can open DevTools
  (Ctrl+Shift+I) and verify Alt+drag and double-click actually fire.
- promptInline / confirmInline kept as advanced fallback for the
  right-click 'change shape' path; the toolbar is now the primary
  interaction surface.
- Preview-render pane now shows a static info card explaining that the
  canvas on the left is the rendered chart (previously blank).
- 6 new tests in tests/flowchart-controller.test.js — toolbar hidden
  by default, selecting a node populates shape buttons + label input +
  Delete, clicking a shape button updates the kind, label input is
  debounced, edge selection populates edge-kind buttons, Delete button
  removes the selected node.

Total tests: 1127 passing (was 1121; +6 new). The 11 pre-existing
PDFOperations failures are unrelated to this change.

package.json

@@ -1,6 +1,6 @@
 {
   "name": "markdown-converter",
-  "version": "4.9.9",
+  "version": "4.10.0",
   "description": "Professional Markdown editor and universal file converter with PDF editing, batch processing, and syntax highlighting",
   "main": "src/main.js",
   "scripts": {