fix: restore transparent background on browser tab icon

- app/icon.png and app/favicon.ico were opaque black since v0.10.4; PWA
  install icons need that (Android auto-fills transparency with white),
  but browser tabs don't, so black looked like a bug there
- re-derived from public/brand/logo-symbol.png, keeping the same ~3.2%
  rightward optical shift used on the other app icons

Author: Randy-R-code