fix(web): render the config form from the schema the save route validates

The form read config_schema.json with a raw json.load while
api_v3.save_plugin_config went through SchemaManager. Those are not the
same schema: SchemaManager applies expand_style_elements, which turns a
compact customization.x-style-elements declaration into the per-element
blocks the form knows how to render.

Without it, that customization object has an x-style-elements key and no
"properties", so the template's object branch matched nothing and the
section rendered as empty space -- while saving still validated against
the expanded shape. of-the-day ships the compact form, so its
customization section has been invisible in the web UI.

pages_v3 gains a schema_manager the way it already has config_manager and
plugin_manager. use_cache=False matches the save route, so an edited
schema is not served stale during plugin development. The raw read stays
as a fallback for callers that register this blueprint without one.

Checked before making the change: load_schema does nothing here except
read, validate and expand -- inject_skin_selector is a separate method it
does not call -- so this is not a behaviour change for schemas without
the declaration.

The test pair renders the same compact schema with and without a
SchemaManager, so it documents exactly what was broken as well as what is
fixed.

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

web_interface/app.py

@@ -163,6 +163,7 @@ pages_v3.config_manager = config_manager
 pages_v3.plugin_manager = plugin_manager
 pages_v3.plugin_store_manager = plugin_store_manager
 pages_v3.saved_repositories_manager = saved_repositories_manager
+pages_v3.schema_manager = schema_manager
 
 api_v3.config_manager = config_manager
 api_v3.plugin_manager = plugin_manager