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/logo-horizontal.svg

@@ -0,0 +1,25 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 720 180" role="img" aria-label="Markdown Converter">
+  <title>Markdown Converter</title>
+  <defs>
+    <linearGradient id="lg" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0%" stop-color="#FB923C"/>
+      <stop offset="100%" stop-color="#C2410C"/>
+    </linearGradient>
+  </defs>
+  <!-- Mark, scaled and translated to fit left side -->
+  <g transform="translate(20, 20) scale(1.4)">
+    <g fill="none" stroke="url(#lg)" 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>
+  </g>
+  <!-- Wordmark -->
+  <g font-family="-apple-system, BlinkMacSystemFont, 'Segoe UI', 'Inter', 'Helvetica Neue', Arial, sans-serif">
+    <text x="180" y="100" font-size="58" font-weight="700" fill="#0F172A" letter-spacing="-1.5">Markdown</text>
+    <text x="424" y="100" font-size="58" font-weight="400" fill="#475569" letter-spacing="-1.2">Converter</text>
+    <text x="182" y="138" font-family="'SF Mono', 'JetBrains Mono', Menlo, Consolas, monospace" font-size="16" font-weight="500" fill="#94A3B8" letter-spacing="4.5">PANDOC &#215; ELECTRON</text>
+  </g>
+</svg>