icons: per-platform icon set — real Liquid Glass on macOS, flat design on Windows/Linux

The hand-packed icns was pixel-clean (verified by unpacking and scanning
every size), but macOS 26 composites legacy .icns icons into the Liquid
Glass system at render time; stacked on Icon Composer's baked PNG-export
lighting, that clips the rim highlights into the near-corner specks and
abrupt highlight ends (owner report 2026-10-13). The fix is the real
layered icon: mac.icon now points at the Icon Composer document
(build/icon.icon, copied from ~/Documents/Affinity/lh-logo) and
electron-builder compiles it via actool into Assets.car — macOS 26+
renders the true Liquid Glass design (dark/tinted variants included).

actool's backwards-compat icns caps at 256px, so scripts/after-pack.cjs
swaps in the existing full-res build/icon.icns before signing: pre-Tahoe
macOS keeps today's quality, Tahoe gets the layered icon.

Windows/Linux get the owner's flat design (rounded squircle with baked
transparency): scripts/make-icons.mjs renders build/icon-flat.svg via
resvg into build/icon.ico (16→256 PNG entries) + build/icon.png (512),
and electron-builder.yml pins win.icon/linux.icon.

desktop:dist injects DEVELOPER_DIR (actool needs Xcode; the machine
selects CommandLineTools by default, CI's setup-xcode takes precedence
via the :- default). The release workflow's mac job selects Xcode 26+
explicitly. Verified locally: mac --dir build (Assets.car + checksummed
full-res fallback + CFBundleIconName/IconFile) and the win-unpacked exe
carries our ico bytes; NSIS/AppImage assembly is CI's (cross-build
tooling can't run on arm64 macOS). Runbook updated. 1610/1610.

.github/workflows/release.yml

@@ -113,6 +113,15 @@ jobs:
         with:
           node-version: "26"
           cache: npm
+      # The mac icon is the Icon Composer document (build/icon.icon);
+      # compiling it needs actool ≥ 26 (Xcode 26+). macos-15 images carry
+      # several Xcodes with an older default — select the latest stable
+      # explicitly. No-op for the windows/linux matrix legs.
+      - name: Select Xcode 26+ for actool (mac icon compile)
+        if: runner.os == 'macOS'
+        uses: maxim-lobanov/setup-xcode@v1
+        with:
+          xcode-version: latest-stable
       - run: npm ci
       - name: Native renderer tests on this OS
         run: npx electron --import tsx --test tests/pdf-native-render.test.ts tests/pdf-ops.test.ts tests/svg-native-render.test.ts tests/browser-service.test.ts