feat: add custom themes and enable Gravatar for Gitea

- Add 21 custom themes from sainnhe/gitea-themes:
  - Edge (auto, dark, light)
  - Everforest (auto, dark, light)
  - Gruvbox (auto, dark, light)
  - Gruvbox Material (auto, dark, light)
  - Nord, Palenight, Soft Era
  - Sonokai (base + 5 variants)
- Enable Gravatar for user avatars
- Set default theme to auto (system preference)
- Mount themes directory to Gitea container

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

dev/compose.yaml

@@ -11,6 +11,7 @@ services:
       - gitea_config:/etc/gitea
       - /etc/timezone:/etc/timezone:ro
       - /etc/localtime:/etc/localtime:ro
+      - ./dev/themes:/data/gitea/public/assets/css:ro
     environment:
       TZ: ${TIMEZONE:-Europe/Berlin}
       USER_UID: 1000
@@ -39,6 +40,12 @@ services:
       GITEA__service__REQUIRE_SIGNIN_VIEW: false
       GITEA__packages__ENABLED: true
       GITEA__actions__ENABLED: true
+      GITEA__ui__THEMES: gitea-auto,gitea-light,gitea-dark,auto,edge-auto,edge-dark,edge-light,everforest-auto,everforest-dark,everforest-light,gruvbox-auto,gruvbox-dark,gruvbox-light,gruvbox-material-auto,gruvbox-material-dark,gruvbox-material-light,nord,palenight,soft-era,sonokai,sonokai-andromeda,sonokai-atlantis,sonokai-espresso,sonokai-maia,sonokai-shusia
+      GITEA__ui__DEFAULT_THEME: auto
+      GITEA__ui__ENABLE_FEED: true
+      GITEA__picture__ENABLE_FEDERATED_AVATAR: true
+      GITEA__picture__AVATAR_UPLOAD_PATH: /data/gitea/avatars
+      GITEA__picture__GRAVATAR_SOURCE: gravatar
     networks:
       - compose_network
     labels: