[Feature]: Themed piece set packs

[BilgeGates]

What problem does this solve?

All current piece sets follow standard western visuals. Users who want a minimalist, illustrated, or retro aesthetic have no alternative — the app ships only Lichess sets with no variety in visual style.

Proposed solution

Ship additional themed piece set packs (e.g. minimalist, illustrated, retro) selectable from the piece set picker alongside existing sets. Packs are curated and bundled — no upload required.

Context

Sub-issue of #162 — custom and fairy piece sets. Scope: curated themed packs only; user-uploaded sets are tracked in #191.

Before submitting

• I searched existing requests and this is not a duplicate.