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/css/styles.css

@@ -199,10 +199,48 @@ h1 {
     transition: background 0.3s;
 }
 
-.status-dot.connected {
+.status-dot.connected,
+.status-dot.status-online {
     background: var(--accent);
 }
 
+.status-dot.status-offline {
+    background: var(--error);
+}
+
+/* Folder management */
+.folder-unavailable-badge,
+.folder-disabled-badge {
+    font-size: 0.75rem;
+    padding: 1px 6px;
+    border-radius: 4px;
+    vertical-align: middle;
+    margin-left: 4px;
+}
+
+.folder-unavailable-badge {
+    background: color-mix(in srgb, var(--error) 20%, transparent);
+    color: var(--error);
+}
+
+.folder-disabled-badge {
+    background: color-mix(in srgb, var(--text-secondary) 20%, transparent);
+    color: var(--text-secondary);
+}
+
+.browser-item.unavailable,
+.browser-list-item.unavailable {
+    opacity: 0.5;
+    cursor: default;
+}
+
+.path-cell {
+    max-width: 250px;
+    overflow: hidden;
+    text-overflow: ellipsis;
+    white-space: nowrap;
+}
+
 .header-toolbar {
     display: flex;
     align-items: center;