feat(packaging): add macOS icon.icns + build:icon-icns script

electron-builder's mac block declared 'icon: assets/icon.icns' but the
file was never generated, so macOS builds shipped with the default
Electron icon. The build would succeed but the .app looked generic.

Add scripts/build-icon-icns.js — a 50-line node script that packs the
existing PNGs from assets/icons/ (16/32/64/128/256/512/1024 + 48 small
+ 16/32/128/256 @2x retina) into a valid ICNS file. No native deps;
works on Linux, mac, and Windows runners alike (avoids the mac-only
'sips' that png2icns requires).

Wired into npm via 'npm run build:icon-icns'. Local build:linux now
parses the ICNS correctly and produces AppImage/deb/snap.

package.json

@@ -32,7 +32,8 @@
     "dist": "electron-builder --publish=never",
     "dist:all": "electron-builder -mwl",
     "download-tools": "node scripts/download-tools.js",
-    "generate-icons": "node scripts/generate-icons.js"
+    "generate-icons": "node scripts/generate-icons.js",
+    "build:icon-icns": "node scripts/build-icon-icns.js"
   },
   "keywords": [
     "markdown",

scripts/build-icon-icns.js

@@ -0,0 +1,75 @@
+/**
+ * Build assets/icon.icns from the PNGs in assets/icons/.
+ *
+ * ICNS file format (Apple Icon Image):
+ *   Header:  'icns' (4 bytes) + total file length (uint32 BE, includes header)
+ *   For each size:
+ *     Type tag (4 bytes) + length (uint32 BE, includes the 8-byte entry header) + PNG data
+ *
+ * Standard PNG type tags we'll include:
+ *   icp4 = 16x16,    icp5 = 32x32,    icp6 = 64x64
+ *   ic07 = 128x128,  ic08 = 256x256,  ic09 = 512x512,  ic10 = 1024x1024
+ *   icsb = 48x48 (small)
+ *   ic11 = 16x16@2x (32x32 retina),  ic12 = 32x32@2x (64x64 retina)
+ *   ic13 = 128x128@2x (256x256 retina), ic14 = 256x256@2x (512x512 retina)
+ */
+const fs = require('fs');
+const path = require('path');
+
+const iconsDir = path.join(__dirname, '..', 'assets', 'icons');
+const outFile = path.join(__dirname, '..', 'assets', 'icon.icns');
+
+// (size-in-pixels, icns type tag) — we skip sizes we don't have on disk
+const sizes = [
+  { size: 16,  type: 'icp4' },
+  { size: 32,  type: 'icp5' },
+  { size: 64,  type: 'icp6' },
+  { size: 128, type: 'ic07' },
+  { size: 256, type: 'ic08' },
+  { size: 512, type: 'ic09' },
+  { size: 1024, type: 'ic10' },
+  { size: 48,  type: 'icsb' },
+  { size: 32,  type: 'ic11' }, // 16@2x
+  { size: 64,  type: 'ic12' }, // 32@2x
+  { size: 256, type: 'ic13' }, // 128@2x
+  { size: 512, type: 'ic14' }, // 256@2x
+];
+
+const entries = [];
+for (const { size, type } of sizes) {
+  const pngPath = path.join(iconsDir, `${size}x${size}.png`);
+  if (!fs.existsSync(pngPath)) continue;
+  const png = fs.readFileSync(pngPath);
+  // ICNS requires 8-byte alignment per entry. paddedLen is the TOTAL entry size
+  // (8-byte type+length header + PNG + padding), rounded up to a multiple of 8.
+  const paddedLen = 8 + Math.ceil(png.length / 8) * 8;
+  entries.push({ type, png, paddedLen });
+  console.log(`  ${type} (${size}x${size}) — ${png.length} bytes`);
+}
+
+if (entries.length === 0) {
+  console.error('No source PNGs found in', iconsDir);
+  process.exit(1);
+}
+
+const totalLen = 8 + entries.reduce((s, e) => s + e.paddedLen, 0);
+const buf = Buffer.alloc(totalLen);
+let off = 0;
+
+// Header
+buf.write('icns', off, 4, 'ascii'); off += 4;
+buf.writeUInt32BE(totalLen, off); off += 4;
+
+// Entries
+for (const { type, png, paddedLen } of entries) {
+  buf.write(type, off, 4, 'ascii'); off += 4;
+  buf.writeUInt32BE(paddedLen, off); off += 4;
+  png.copy(buf, off);
+  off += png.length;
+  // Pad to 8-byte boundary within the entry. paddedLen already includes the 8-byte header.
+  const pad = paddedLen - 8 - png.length;
+  if (pad > 0) off += pad;
+}
+
+fs.writeFileSync(outFile, buf);
+console.log(`\nWrote ${outFile} (${buf.length} bytes, ${entries.length} sizes)`);