refactor(web): drop the two dead client-side config-form renderers

generateConfigForm and generateSimpleConfigForm (580 lines) were defined
on the Alpine component and never called: server-side Jinja replaced them,
as pages_v3.py:641 records. Nothing in any template invokes them -- there
is no x-html in the templates and no bracket access on the component.

They carried their own x-widget dispatch, which made them an active trap:
the next person adding a widget would reasonably think both renderers
needed updating.

plugins/config_manager.js (PluginConfigManager, 133 lines) goes for the
same reason -- loaded on every page from base.html, referenced only by
itself and by an archived doc.

Kept, having checked them: widgets/example-color-picker.js is the worked
example docs/widget-guide.md points plugin authors at, and
widgets/plugin-loader.js is the client half of a documented feature
(manifest-declared plugin widgets) whose server route is missing --
soccer-scoreboard already ships a widgets/custom-leagues.js that this
loader is meant to fetch. That is an unfinished feature to complete, not
dead code to delete.

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

web_interface/templates/v3/base.html

@@ -972,7 +972,6 @@
     <!-- Load plugin management modules -->
     <script src="{{ url_for('static', filename='v3/js/plugins/store_manager.js') }}" defer></script>
     <script src="{{ url_for('static', filename='v3/js/plugins/state_manager.js') }}" defer></script>
-    <script src="{{ url_for('static', filename='v3/js/plugins/config_manager.js') }}" defer></script>
     <script src="{{ url_for('static', filename='v3/js/plugins/install_manager.js') }}" defer></script>
     <!-- Shared search/filter/sort controller for the plugin card grids -->
     <script src="{{ url_for('static', filename='v3/js/plugins/list_filter.js') }}" defer></script>