feat(folder-browser): Miller columns directory navigation

New page /folders with macOS Finder-style column layout:
- getMusicDirectory / getMusicIndexes / getMusicFolders Subsonic API methods
- Root level uses getIndexes.view (music folder IDs are not getMusicDirectory IDs)
- Columns fill available width with flex: 1, min-width: 200px
- Auto-scroll to rightmost column on expand
- Cancelled-flag prevents stale state on fast navigation
- Clicking a track plays it in the context of the current column's tracks
- FolderOpen/Folder/Music icons, accent-colored selection, ellipsis truncation
- Hidden in sidebar by default (user can enable in Settings)
- i18n: EN DE FR NL NB ZH RU

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

src/styles/components.css

@@ -6767,3 +6767,111 @@
   transform: scale(1.2);
   box-shadow: 0 0 0 4px color-mix(in srgb, var(--accent) 20%, transparent);
 }
+
+/* ── Folder Browser (Miller Columns) ──────────────────────────────────────── */
+
+.folder-browser {
+  display: flex;
+  flex-direction: column;
+  padding: 0;
+  height: 100%;
+}
+
+.folder-browser-title {
+  padding: 1.5rem 1.5rem 0.75rem;
+  flex-shrink: 0;
+}
+
+.folder-browser-columns {
+  flex: 1;
+  min-height: 0;
+  display: flex;
+  flex-direction: row;
+  overflow-x: auto;
+  overflow-y: hidden;
+  /* height fallback for browsers that don't support flex: 1 correctly here */
+  height: calc(100vh - var(--player-height, 88px) - var(--titlebar-height, 0px) - 70px);
+  border-top: 1px solid var(--border-subtle);
+}
+
+.folder-browser-columns::-webkit-scrollbar {
+  height: 6px;
+}
+
+.folder-col {
+  flex: 1;
+  min-width: 200px;
+  height: 100%;
+  overflow-y: auto;
+  overflow-x: hidden;
+  border-right: 1px solid var(--border-subtle);
+}
+
+.folder-col-status {
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  height: 80px;
+  color: var(--text-muted);
+  font-size: 0.8rem;
+  padding: 1rem;
+  text-align: center;
+}
+
+.folder-col-error {
+  color: var(--danger, #f38ba8);
+}
+
+.folder-col-row {
+  display: flex;
+  align-items: center;
+  gap: 0.5rem;
+  width: 100%;
+  padding: 0.38rem 0.6rem 0.38rem 0.75rem;
+  background: none;
+  border: none;
+  border-radius: 0;
+  cursor: pointer;
+  color: var(--text-primary);
+  font-size: 0.8rem;
+  text-align: left;
+  transition: background 0.1s;
+}
+
+.folder-col-row:hover {
+  background: var(--bg-hover, color-mix(in srgb, var(--accent) 8%, transparent));
+}
+
+.folder-col-row.selected {
+  background: var(--accent);
+  color: #fff;
+}
+
+.folder-col-row.selected .folder-col-chevron {
+  color: rgba(255, 255, 255, 0.7);
+}
+
+.folder-col-icon {
+  flex-shrink: 0;
+  opacity: 0.75;
+  display: flex;
+  align-items: center;
+}
+
+.folder-col-row.selected .folder-col-icon {
+  opacity: 1;
+}
+
+.folder-col-name {
+  flex: 1;
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+}
+
+.folder-col-chevron {
+  flex-shrink: 0;
+  color: var(--text-muted);
+  margin-left: auto;
+}