docs: fix stale Icon Generation block + close spec drift on tray-icon@2x

Final-pass review found two doc misses:

(1) docs/systems/desktop.md had a SECOND 'Icon Generation' section
under Build System (separate from the Tray Icon section that Task 9
updated) that still described the deleted scripts/gen-icns.sh —
sips/iconutil pipeline using present tense. Replaced with an accurate
description of the new SVG-driven scripts/gen-icons.mjs pipeline,
including the dev-time Electron Resources cp note and links to the
spec and generator README.

(2) The icon-system spec's 'Files Deleted' table only listed
/icon.png and gen-icns.sh; tray-icon@2x.png was deleted in Task 8 but
not documented. Added with the Cocoa-template-only rationale.

Doc-only fix; no code changes.

docs/systems/desktop.md

@@ -751,11 +751,11 @@ output: dist-electron
 
 ### Icon Generation
 
-**`scripts/gen-icns.sh`** — macOS-only, generates `.icns` from `icon.png` via:
-1. `sips` to resize into all required icon sizes (16-1024px, including @2x variants)
-2. `iconutil -c icns` to pack the iconset
+All desktop and web brand assets are generated by `scripts/gen-icons.mjs` from three SVG sources in `assets/brand/` (`app-icon.svg`, `mark.svg`, `mark-mono-dark.svg`). Run via `pnpm gen-icons` after artwork changes; commit the diff. The generator uses `sharp` (SVG → PNG), `png-to-ico` (multi-size `.ico`), and `png2icons` (`.icns`). Output is byte-stable for a given lockfile.
 
-Used in `pnpm dev` to set the dev Electron icon.
+The `dev` script copies the committed `build/icon.icns` into Electron's bundled `Resources/electron.icns` so the macOS dev dock shows the Backspace mark instead of the default Electron logo. This dev-only patch is independent of how `.icns` is generated.
+
+See [the icon system spec](../superpowers/specs/2026-04-27-icon-system-design.md) for the full output matrix and `scripts/gen-icons.README.md` for the regeneration workflow and version-bump caveat.
 
 ### Auto-Update Publishing