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.

assets/markdown-converter-assets/app-icon-dark.svg

@@ -0,0 +1,27 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 1024 1024" role="img" aria-label="Markdown Converter app icon (dark)">
+  <title>Markdown Converter (dark)</title>
+  <defs>
+    <linearGradient id="darkBg" x1="0" y1="0" x2="0" y2="1024" gradientUnits="userSpaceOnUse">
+      <stop offset="0%" stop-color="#1C1917"/>
+      <stop offset="100%" stop-color="#0C0A09"/>
+    </linearGradient>
+    <radialGradient id="darkGlow" cx="280" cy="180" r="780" gradientUnits="userSpaceOnUse">
+      <stop offset="0%" stop-color="#EA580C" stop-opacity="0.65"/>
+      <stop offset="60%" stop-color="#9A3412" stop-opacity="0.15"/>
+      <stop offset="100%" stop-color="#9A3412" stop-opacity="0"/>
+    </radialGradient>
+    <linearGradient id="darkMark" x1="0" y1="180" x2="0" y2="844" gradientUnits="userSpaceOnUse">
+      <stop offset="0%" stop-color="#FFEDD5"/>
+      <stop offset="100%" stop-color="#FB923C"/>
+    </linearGradient>
+  </defs>
+  <rect width="1024" height="1024" rx="224" ry="224" fill="url(#darkBg)"/>
+  <rect width="1024" height="1024" rx="224" ry="224" fill="url(#darkGlow)"/>
+  <g transform="translate(154, 176) scale(7.16)" fill="none" stroke="url(#darkMark)" stroke-width="9" stroke-linecap="round" stroke-linejoin="round">
+    <line x1="26" y1="22" x2="26" y2="72"/>
+    <line x1="63" y1="22" x2="63" y2="75"/>
+    <polyline points="22,18 44.5,56 67,18"/>
+    <polyline points="48,60 63,76 78,60"/>
+  </g>
+</svg>