refactor(flowchart): build script generates bundle from pure modules (D1)

Before this commit, src/renderer/flowchart-bundle.js was a 1700+
line hand-maintained file with ~1100 lines of inlined copies of the
pure modules (shapes, mermaid, mermaid-parse, store, canvas,
clipboard, align). Every change to a pure module required a manual
re-sync, and the C17 multi-select commit had to be applied in two
places — drift between source and bundle was a real risk.

This refactor introduces scripts/build-flowchart-bundle.js that
reads each pure module from src/flowchart/ and concatenates them
into the final src/renderer/flowchart-bundle.js that the standalone
window loads as a single <script> tag.

Architecture:

  - src/renderer/flowchart-bundle.js — now auto-generated, gitignored
    only conceptually (still tracked so the standalone window works
    without running a build step in dev).
  - src/renderer/flowchart-bundle-tail.js — the unique code that
    was previously buried in the middle of the hand-maintained bundle:
    IIFE open, defensive checks, pure-module destructure, controller
    bootstrap, modal helpers, node-list panel, closing IIFE, and
    DOMContentLoaded bootstrap trigger.
  - scripts/build-flowchart-bundle.js — the generator. Reads 8 pure
    modules in dependency order (vsdx-export excluded — main-process
    only), wraps each in its own IIFE for scope isolation, rewrites
    canvas's CommonJS require() lines to use window globals, and
    rewrites bare 'module.exports = X' to 'window.FlowchartX = X'
    for modules without UMD guards.

Tests: 1386/1386 passing (no regressions). The pre-existing per-node
fill color test that was passing before now passes for the right
reason — the canvas source was updated to pass node.color through
to shapeSvg, which it always should have. The hand-maintained bundle
had been silently carrying a patched version that passed color but
was never backported to src/.

Amit Haridas

package.json

@@ -29,6 +29,7 @@
     "dist:all": "electron-builder --config electron-builder.config.js -mwl",
     "download-tools": "node scripts/download-tools.js",
     "bundle:markitdown": "node scripts/bundle-markitdown.js",
+    "build:bundle": "node scripts/build-flowchart-bundle.js",
     "generate-icons": "node scripts/generate-icons.js"
   },
   "keywords": [