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.

src/welcome.js

@@ -11,6 +11,7 @@ function createWelcomeContent(recentFiles = [], appVersion = '') {
   return `
     <div class="welcome-container">
         <div class="welcome-hero">
+            <img src="../assets/markdown-converter-assets/mark.svg" alt="" width="56" height="56" class="welcome-mark" />
             <h1 class="welcome-title">MarkdownConverter</h1>
             <p class="welcome-version">${appVersion ? `Version ${appVersion}` : ''}</p>
             <p class="welcome-subtitle">Professional Markdown Editor & Universal Document Converter</p>