feat(import): embed Microsoft MarkItDown for any-file → Markdown import

- File → Import with MarkItDown (Any Format)…: PDF, DOCX, PPTX, XLSX,
  Outlook .msg/.eml, EPUB, images, CSV/JSON/XML, ZIP (audio/OCR via the
  [all] extras) — verified live against HTML, XLSX (our own exporter's
  output), and PDF fixtures
- Command auto-resolution with caching: markitdown binary → python -m
  markitdown → python3 -m markitdown
- SEC-1 argv discipline (execFile only, user paths never through a shell),
  50MB cap, 120s timeout, sanitized errors that surface markitdown's own
  "pip install 'markitdown[pdf]'" hints for missing format extras
- Output lands next to the source as <name>.md (numeric suffix, never
  overwrites) and opens in a new tab; markitdown:available/convert IPC
  allowlisted for renderer flows
- Help → Dependencies lists MarkItDown; README/UPDATES updated (v4.6.1)

12 new tests (629 green); lint clean; clean app boot

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "markdown-converter",
-  "version": "4.6.0",
+  "version": "4.6.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "markdown-converter",
-      "version": "4.6.0",
+      "version": "4.6.1",
       "license": "MIT",
       "dependencies": {
         "@cantoo/pdf-lib": "^2.9.1",