feat(icons): rebrand app icon; slice the full set from one master

The SVG tracer export failed, so the icon was designed collaboratively from
scratch: a wooden «Э» tile with a ✻ score-subscript in Spectral Bold, wood
grain and diagonal light/shadow. Construction is specified in fractions of the
side in docs/ICON_BRANDBOOK.md; the reference generator, pinned Spectral font
and committed masters live in assets/icons/brand/.

One master -> the whole set (assets/icons/brand/build-set.mjs + build-android-res.mjs):
- web (ui/public): favicon.svg (light+dark via prefers-color-scheme), favicon.ico,
  apple-touch, PWA any + maskable (+ dark variants), og-image reskinned to
  «Эрудит» / Игра в слова
- Capacitor layers: ui/assets/{icon,icon-foreground,icon-background}.png
- Android launcher res, all densities, NO inset, + monochrome themed layer
  (anydpi xml: background+foreground+monochrome)
- manual-upload art: assets/icons/brand/{vk,tg,store} (square, no rounding)

Primary variant light; dark where the platform can theme (favicon.svg). The old
LiberationSans generator (assets/icons/build) is removed; docs/ICONS.md retargeted
onto the single master. ANDROID_PLAN icon-rebrand item marked done.

assets/icons/brand/.gitignore

@@ -0,0 +1,2 @@
+node_modules/
+package-lock.json