feat: add media folder management from WebUI

- Add media_folders_management config flag (enabled by default)
- Guard folder CRUD endpoints with 403 when management disabled
- Wire up frontend folder add/edit/delete in Settings tab
- Add per-folder availability check (for network shares)
- Show unavailable badge on offline folders in browser view
- Expose management flag via /api/health endpoint
- Add EN/RU locale keys for folder management UI

media_server/static/index.html

@@ -323,6 +323,39 @@
                 </div>
             </details>
 
+            <details class="settings-section" open id="mediaFoldersSection" style="display: none;">
+                <summary data-i18n="settings.section.media_folders">Media Folders</summary>
+                <div class="settings-section-content">
+                    <p class="settings-section-description" data-i18n="browser.folders_description">
+                        Media folders available for browsing. Folders on network shares show availability status.
+                    </p>
+                    <table class="scripts-table">
+                        <thead>
+                            <tr>
+                                <th data-i18n="browser.folders_table.id">ID</th>
+                                <th data-i18n="browser.folders_table.label">Label</th>
+                                <th data-i18n="browser.folders_table.path">Path</th>
+                                <th data-i18n="browser.folders_table.status">Status</th>
+                                <th data-i18n="browser.folders_table.actions">Actions</th>
+                            </tr>
+                        </thead>
+                        <tbody id="foldersTableBody">
+                            <tr>
+                                <td colspan="5" class="empty-state">
+                                    <div class="empty-state-illustration">
+                                        <svg viewBox="0 0 64 64"><path d="M8 16h20l4-6h20a4 4 0 014 4v36a4 4 0 01-4 4H8a4 4 0 01-4-4V20a4 4 0 014-4z"/><path d="M4 24h56" stroke-dasharray="4 3" opacity="0.4"/></svg>
+                                        <p data-i18n="browser.folders_empty">No media folders configured. Click "+" to add one.</p>
+                                    </div>
+                                </td>
+                            </tr>
+                        </tbody>
+                    </table>
+                    <div class="add-card" onclick="showAddFolderDialog()">
+                        <span class="add-card-icon">+</span>
+                    </div>
+                </div>
+            </details>
+
             <details class="settings-section" open>
                 <summary data-i18n="settings.section.scripts">Scripts</summary>
                 <div class="settings-section-content">