feat: civ-style hex map with Kenney HD tiles + UI expand modals (#13)

Replace procedural cartoon textures with Kenney Hexagon Pack 4x tiles
(480×561 px, CC0). Rewrite hex rendering to use pointy-top grid with
culture borders, territory color overlays, and per-owner roof recoloring.
Add click-to-expand Portal modals for Memories, Inbox, Chronicle,
Combat Log, and World Events panels.

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: keanji-x