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/display.py

@@ -25,17 +25,12 @@ def get_display_current():
 
         snapshot_path = "/tmp/led_matrix_preview.png"
 
-        # Get display dimensions from config
+        # Get display dimensions from config: the logical size DisplayManager
+        # renders at, so double-sided setups preview one screen
         try:
             if api_v3.config_manager:
-                main_config = api_v3.config_manager.load_config()
-                hardware_config = main_config.get('display', {}).get('hardware', {})
-                cols = hardware_config.get('cols', 64)
-                chain_length = hardware_config.get('chain_length', 2)
-                rows = hardware_config.get('rows', 32)
-                parallel = hardware_config.get('parallel', 1)
-                width = cols * chain_length
-                height = rows * parallel
+                from src.display_geometry import logical_size
+                width, height = logical_size(api_v3.config_manager.load_config())
             else:
                 width = 128
                 height = 64