chore: mark skins unsupported, fix stale docs and preview size, prepare 3.4.0

Skins: no current scoreboard plugin builds on src.base_classes, so the only
skin hook (SportsCore._render_game) never runs. The plugin schema endpoint no
longer injects the Visual Skin dropdown, the store hides and refuses
"type": "skin" registry entries, and GET /api/v3/skins reports
supported: false with a message. Stored skin config still loads and saves.
src/skin_system/ and its tests are unchanged apart from the support flag.

Docs: check_plugin.py/render_plugin.py examples use --plugin; document
BasePlugin.get_update_interval() and its interaction with the manifest
update_interval; CLAUDE.md drops the stale template line number and
recommends display_manager.width/height.

Preview size: new src/display_geometry.py holds the size computation and
defaults DisplayManager uses (double-sided applied, chain_length default 2).
The web preview, /display/current, Starlark magnify default, sync handshake
and two dev scripts use it.

Release: __version__ 3.4.0, CHANGELOG 3.4.0 section plus a 3.3.0 tag note.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

web_interface/blueprints/api_v3/misc.py

@@ -154,9 +154,15 @@ def list_skins():
     """List installed visual skins (docs/SKIN_SYSTEM.md).
 
     Optional ?plugin_id=... filters to skins matching that plugin.
+
+    The response carries ``supported: false`` and a ``message``: the current
+    scoreboard plugins don't render skins, so a client must not present
+    these as selectable.
     """
     try:
-        from src.skin_system import skin_runtime
+        from src.skin_system import (
+            SKINS_RENDER_SUPPORTED, SKINS_UNSUPPORTED_MESSAGE, skin_runtime,
+        )
 
         plugin_id = request.args.get('plugin_id')
         if plugin_id:
@@ -181,7 +187,10 @@ def list_skins():
                 'modes': manifest.get('modes', []),
                 'has_preview': bool(preview and (skin_dir / preview).is_file()),
             })
-        return jsonify({'status': 'success', 'data': {'skins': payload}})
+        data = {'skins': payload, 'supported': SKINS_RENDER_SUPPORTED}
+        if not SKINS_RENDER_SUPPORTED:
+            data['message'] = SKINS_UNSUPPORTED_MESSAGE
+        return jsonify({'status': 'success', 'data': data})
     except Exception as e:
         logger.error('Error in list_skins', exc_info=True)
         return jsonify({'status': 'error', 'message': 'An error occurred; see logs for details', 'details': describe_exception(e)}), 500