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>

src/plugin_system/store_manager.py

@@ -1338,9 +1338,16 @@ class PluginStoreManager:
             self.logger.error(f"Plugin not found in registry: {plugin_id}")
             return False
 
-        # Visual skins share the registry but install to skins/, not to a
-        # plugin directory (docs/SKIN_SYSTEM.md)
+        # Visual skins share the registry. _install_skin_from_info can put one
+        # in skins/, but no current scoreboard plugin renders skins, so the
+        # store refuses them rather than installing something that does
+        # nothing (docs/SKIN_SYSTEM.md). Manual installs under skins/ and
+        # uninstall_skin are unaffected.
         if (plugin_info.get('type') or 'plugin') == 'skin':
+            from src.skin_system import SKINS_RENDER_SUPPORTED, SKINS_UNSUPPORTED_MESSAGE
+            if not SKINS_RENDER_SUPPORTED:
+                self.logger.error(f"Not installing skin {plugin_id}: {SKINS_UNSUPPORTED_MESSAGE}")
+                return False
             return self._install_skin_from_info(plugin_id, plugin_info, branch)
 
         repo_url = plugin_info.get('repo')