feat(flowchart): Open from file button + parse on load (C9)

v4.13.0 — round-trips Mermaid source through the editor:

  - new open-text-file-dialog invoke IPC in main.js opens a system
    Open dialog filtered for .mmd / .mermaid / .md / .markdown / .txt,
    reads the chosen file, returns { path, content } or null on cancel.
  - new openFile() on the flowchart preload bridge.
  - new Open from File button in the standalone window toolbar
    alongside Insert / Save / Reset.
  - On click: confirms overwrite when the current graph is non-empty,
    strips any mermaid fence, parses via the inlined fromMermaid()
    (the inverse of toMermaid), and calls _store.deserialize so the
    canvas redraws. Status bar reports Loaded path.

The fromMermaid() in the bundle is duplicated from
src/flowchart/flowchart-mermaid-parse.js (23 tests in C8 land the
same logic) so the standalone window does not depend on the bundle
rebuild step. Pure module + bundle inline stay in sync logically;
future work could collapse them via a build-time inclusion.

Amit Haridas

src/flowchart-generator.html

@@ -384,6 +384,13 @@
             >
               Save to File
             </button>
+            <button
+              class="fc-btn"
+              id="fc-btn-open"
+              title="Open a .mmd / .md file and load its flowchart"
+            >
+              Open from File
+            </button>
             <button class="fc-btn" id="fc-btn-reset" title="Clear all nodes and edges">
               Reset All
             </button>