Merge origin/main into claude/remove-skins-and-base-classes

Resolves the README.md / docs/CONFIG_REFERENCE.md conflicts with #607 (its
corrected text and function-name references, without the skins section; this
branch's use_short_date_format row), and drops the skin/skin_options entries
#607 documented in PLUGIN_CONFIG_CORE_PROPERTIES.md, since this branch retires
those keys.

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

test/fixtures/api_v3_url_map.json

@@ -358,22 +358,6 @@
       "POST"
     ]
   ],
-  [
-    "/api/v3/plugins/authenticate/spotify",
-    "api_v3.authenticate_spotify",
-    [
-      "OPTIONS",
-      "POST"
-    ]
-  ],
-  [
-    "/api/v3/plugins/authenticate/ytm",
-    "api_v3.authenticate_ytm",
-    [
-      "OPTIONS",
-      "POST"
-    ]
-  ],
   [
     "/api/v3/plugins/calendar/authenticate",
     "api_v3.authenticate_calendar",
@@ -511,22 +495,6 @@
       "POST"
     ]
   ],
-  [
-    "/api/v3/plugins/of-the-day/json/delete",
-    "api_v3.delete_of_the_day_json",
-    [
-      "OPTIONS",
-      "POST"
-    ]
-  ],
-  [
-    "/api/v3/plugins/of-the-day/json/upload",
-    "api_v3.upload_of_the_day_json",
-    [
-      "OPTIONS",
-      "POST"
-    ]
-  ],
   [
     "/api/v3/plugins/operation/<operation_id>",
     "api_v3.get_operation_status",