feat(icons): generate Tauri desktop icon set from brand.toml

[IamMrCupp]

Closes #30. Part of #26 (Phase 3 — extension).

The Tauri prototype in audiophore/audiophore ships flat-color placeholder icons — committed once, never the real mark. This generates the actual set its tauri.conf.json icon array expects:

• logos/icons/icon.icns — full macOS set (16/32/128/256/512 + @2x), written by a small pure-Python ICNS writer. No macOS iconutil in the build path (it's absent on the ubuntu CI runner); verified that iconutil itself parses the output.
• logos/icons/icon.ico — multi-res Windows/app icon (16–256), via ImageMagick, the same path favicon.ico already uses.
• logos/icons/{32x32,128x128,128x128@2x}.png — the PNGs the icon array points at directly.

New [icons.desktop] section in brand.toml (source variant + .ico sizes). The .icns is byte-identical on rebuild; the web favicon.ico was never a substitute for a desktop app icon.

Wiring these into tauri.conf.json is a follow-up in the app repo.

[IamMrCupp]

Switched icon.ico from ImageMagick to a pure-Python ICO writer over the cairosvg PNGs. ImageMagick's encoder isn't byte-stable across versions (homebrew IMv7 locally vs the ubuntu runner), which tripped the reproducibility gate; the PNGs themselves are identical cross-platform, so framing them in a deterministic ICO container fixes it. Amended onto the original commit.