feat(git-status): group changed files under collapsible folders (setting, default on)

The Git window shows each group's files under their folders, collapsed until clicked, with single-child folder chains merged and open folders surviving the refresh. App Settings → Bottom bar → 'Git status: group files by folder' (per device) switches back to the flat list.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JrzFKEdBLwVfu6ev2ZscJS

src/web/public/styles.css

@@ -19634,3 +19634,39 @@ html .toolbar .btn-git-status.git-status--conflict {
 .git-diff-line--meta {
   color: var(--text-muted);
 }
+
+/* Git status panel: files grouped under collapsible folders */
+.git-tree-dir > .git-tree-summary {
+  display: flex;
+  align-items: baseline;
+  gap: 0.45rem;
+  padding: 0.15rem 0.25rem;
+  border-radius: 4px;
+  cursor: pointer;
+  min-width: 0;
+}
+
+.git-tree-dir > .git-tree-summary:hover {
+  background: var(--bg-hover);
+}
+
+.git-tree-name {
+  flex: 1;
+  min-width: 0;
+  overflow: hidden;
+  text-overflow: ellipsis;
+  white-space: nowrap;
+  font-family: 'SF Mono', Monaco, monospace;
+  font-size: 0.7rem;
+}
+
+.git-tree-count {
+  font-size: 0.64rem;
+  color: var(--text-muted);
+}
+
+.git-tree-children {
+  margin-left: 0.7rem;
+  padding-left: 0.4rem;
+  border-left: 1px solid var(--border);
+}