feat(brand): adopt the new M↓ brand kit; fix CI tool downloads

Branding:
- All app icons regenerated from the new vector brand kit (M↓ mark):
  icon.png/icon@2x (app + packaging), favicon.png, tray-icon.png, and the
  full assets/icons/ size set — generate-icons.js now rasterizes
  app-icon.svg directly (docico1.png removed)
- index.html gets proper favicon/apple-touch links from the kit
- Welcome tab hero shows the new mark; README gets the horizontal wordmark
- assets/logo.png (ConcreteInfo) intentionally untouched

CI release fixes (win/mac jobs were failing):
- FiraCode download moved from moving raw/master URLs (hash drifted
  upstream, tripping the pin) to the immutable 6.2 release asset; repo
  fonts updated to the pinned 6.2 bits
- macOS pandoc extractor locates the binary in the archive instead of
  assuming a bin/ layout that the macOS zip doesn't have

Stray upload archive (markdown-converter-assets (1).zip) excluded.

package-lock.json

@@ -1,12 +1,12 @@
 {
   "name": "markdown-converter",
-  "version": "4.7.0",
+  "version": "4.7.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
       "name": "markdown-converter",
-      "version": "4.7.0",
+      "version": "4.7.1",
       "license": "MIT",
       "dependencies": {
         "@cantoo/pdf-lib": "^2.9.1",