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/index.html

@@ -10,6 +10,19 @@
     />
     <meta name="viewport" content="width=device-width, initial-scale=1.0" />
     <title>MarkdownConverter</title>
+    <!-- Brand favicons (M↓ mark) -->
+    <link rel="icon" type="image/png" sizes="32x32" href="../assets/favicon.png" />
+    <link
+      rel="icon"
+      type="image/png"
+      sizes="16x16"
+      href="../assets/markdown-converter-assets/favicons/favicon-16x16.png"
+    />
+    <link
+      rel="apple-touch-icon"
+      sizes="180x180"
+      href="../assets/markdown-converter-assets/favicons/apple-touch-icon.png"
+    />
     <!-- Design tokens - loaded first for CSS variable availability -->
     <link rel="stylesheet" href="styles/tokens.css" />
     <link rel="stylesheet" href="styles/modal.css" />