feat(flowchart): button-based UI replaces click-canvas interaction; v4.11.0

Replaced the v4.10.0 floating selection toolbar (which relied on
SVG click hit-testing inside #canvas-host and was unreliable in the
user's Electron runtime) with a button-driven node-list panel
(#fc-nodelist) below the canvas. Every mutation now flows through
explicit controls: 5 Add Node buttons, per-node kind-select +
label-input + delete ×, per-edge kind-select + label-input + delete
×, and a Connect form (From/To selects + Edge button + Refresh).

The canvas is purely visual now: no more #fc-selection-toolbar, no
controller-level _selectedId/_selectedKind state, no keyboard
Delete/Backspace selection handler. Canvas click callbacks
(onNodeClick, onEdgeClick, onShapeMenu) are no-ops. The 5-shape /
3-edge-kind / label-input / delete-button surface is unchanged in
spirit — it just lives in the panel instead of floating over the
canvas.

Files:
- src/flowchart-generator.html: added #fc-nodelist between canvas and
  preview (wrapped in new .fc-left column), removed fc-selection-toolbar,
  bumped header hint.
- src/renderer/flowchart-bundle.js: removed SHAPE_BUTTONS/EDGE_BUTTONS,
  setSelection, renderSelectionToolbar, appendLabelInput,
  appendDeleteButton, the keyboard Delete handler, and the
  controller-level selection state. Added rerenderNodeList +
  shapeLabel/edgeLabel, wired all panel buttons (add-row, per-node
  selects/inputs/deletes, per-edge selects/inputs/deletes, connect
  form). Subscribe now calls rerenderNodeList instead of the old
  renderSelectionToolbar.
- tests/flowchart-controller.test.js: replaced 6 v4.10.0 selection
  toolbar tests with 11 v4.11.0 button-driven panel tests.
- package.json: 4.10.0 -> 4.11.0.
- README.md: version line bumped to v4.11.0.
- UPDATES.md: added v4.11.0 changelog entry.

Test summary: 92 suites, 1143 tests pass (was 1138 — +5 net).
Lint + prettier clean.

Amit Haridas

package.json

@@ -1,6 +1,6 @@
 {
   "name": "markdown-converter",
-  "version": "4.10.0",
+  "version": "4.11.0",
   "description": "Professional Markdown editor and universal file converter with PDF editing, batch processing, and syntax highlighting",
   "main": "src/main.js",
   "scripts": {