refactor: remove the skin system

Skins never rendered with the current scoreboard plugins: the only hook was
SportsCore._render_game in src/base_classes, which no plugin builds on, so
the UI and store already treated them as unsupported. The owner decided on
2026-09-23 to remove them outright.

Removed src/skin_system/ (runtime, base class, fixtures), skins/,
scripts/validate_skin.py and their tests; the store's "type": "skin"
installer, uninstaller and hide/refuse filters (the official registry lists
no skins); SchemaManager.inject_skin_selector; and GET /api/v3/skins.

Stored skin/skin_options config values are handled in the next commit.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

test/fixtures/api_v3_url_map.json

@@ -671,15 +671,6 @@
       "POST"
     ]
   ],
-  [
-    "/api/v3/skins",
-    "api_v3.list_skins",
-    [
-      "GET",
-      "HEAD",
-      "OPTIONS"
-    ]
-  ],
   [
     "/api/v3/starlark/apps",
     "api_v3.get_starlark_apps",