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.

README.md

@@ -1,3 +1,7 @@
+<p align="center">
+  <img src="assets/markdown-converter-assets/logo-horizontal.svg" alt="Markdown Converter" width="420">
+</p>
+
 # MarkdownConverter
 
 A powerful cross-platform Markdown editor and document converter powered by Pandoc, built with Electron. 100% open-source with no proprietary dependencies.
@@ -220,4 +224,4 @@ Amit Haridas (amit.wh@gmail.com)
 
 ## Version
 
-v4.7.0
+v4.7.1