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/static/v3/plugins_manager.js

@@ -3446,6 +3446,23 @@ function generateFieldHtml(key, prop, value, prefix = '') {
             html += `<option value="${option}" ${selected}>${option}</option>`;
         });
         html += `</select>`;
+    } else if (prop['x-widget'] === 'json-file-manager') {
+        // Reusable JSON file manager widget (no CDN, keyboard shortcuts, configurable actions)
+        const widgetConfig = prop['x-widget-config'] || {};
+        const pluginId = currentPluginConfig?.pluginId || window.currentPluginConfig?.pluginId || '';
+        const safeFieldId = (fullKey || 'file_manager').replace(/[^a-zA-Z0-9_-]/g, '_');
+
+        html += `<div id="${safeFieldId}_jfm_mount"></div>`;
+
+        setTimeout(() => {
+            const mount = document.getElementById(`${safeFieldId}_jfm_mount`);
+            if (!mount) return;
+            if (typeof JsonFileManager !== 'undefined') {
+                new JsonFileManager(mount, widgetConfig, pluginId);
+            } else {
+                mount.innerHTML = '<p style="color:#dc2626;font-size:.875rem;">json-file-manager widget not loaded. Check base.html includes json-file-manager.js.</p>';
+            }
+        }, 150);
     } else if (prop['x-widget'] === 'custom-html') {
         // Custom HTML widget - load HTML from plugin directory
         const htmlFile = prop['x-html-file'];