feat(ui): add reusable json-file-manager widget for plugin file management

Introduces JsonFileManager — a zero-CDN, keyboard-accessible, configurable
widget for managing JSON data files from plugin configuration forms.

web_interface/static/v3/js/widgets/json-file-manager.js (new):
- Self-contained class with scoped CSS (no global leakage)
- File list with cards: enable/disable toggle, entry count, size, date
- Drag-and-drop + click-to-browse JSON upload
- Textarea-based JSON editor (no CDN); Format + Validate buttons
- Ctrl+S to save, Escape to close any open modal
- Create-new-file modal with configurable fields and validation
- Delete confirmation modal
- All actions (list/get/save/upload/delete/create/toggle) are configurable
  via x-widget-config in config_schema.json — no plugin-ID hardcoding

web_interface/static/v3/plugins_manager.js:
- New handler for x-widget: "json-file-manager" — renders mount div,
  instantiates JsonFileManager with x-widget-config and plugin ID

web_interface/templates/v3/base.html:
- Include json-file-manager.js (defer) before plugins_manager.js

Usage: set x-widget: "json-file-manager" + x-widget-config in any
plugin's config_schema.json (see ledmatrix-plugins of-the-day for a
complete example).

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web_interface/templates/v3/base.html

@@ -4625,6 +4625,9 @@
     <script src="{{ url_for('static', filename='v3/js/widgets/timezone-selector.js') }}" defer></script>
     <script src="{{ url_for('static', filename='v3/js/widgets/plugin-loader.js') }}" defer></script>
     
+    <!-- Reusable JSON file manager widget (used by of-the-day and others via x-widget: json-file-manager) -->
+    <script src="{{ url_for('static', filename='v3/js/widgets/json-file-manager.js') }}" defer></script>
+
     <!-- Legacy plugins_manager.js (for backward compatibility during migration) -->
     <script src="{{ url_for('static', filename='v3/plugins_manager.js') }}?v=20260307" defer></script>