feat: explorer copy-path & @-reference actions; icons on tabs and + menu

- Tabs, the + menu, and empty-pane cards carry per-type icons (folder /
  branch / terminal / code file); a terminal glyph joins the icon set
- Hovering an explorer row reveals an @文件 button at the far right that
  appends @<relative path> to the composer draft (session-scoped input
  service, space-separated append)
- Right-clicking a row opens a context menu: copy relative / absolute
  path (writeClipboard + transient in-row "copied" label)
- Rows become div role=button (Enter/Space) to host the inner button;
  relativeTo helper unit-tested

src/client/paths.ts

@@ -0,0 +1,23 @@
+/**
+ * Path projection helpers shared by the explorer rows: a path relative to
+ * the session cwd (for the @-reference button and "copy relative path").
+ * The fs-tree joins with '/' even on Windows, so both separators normalize
+ * to '/' before comparison.
+ */
+
+/**
+ * The path relative to the session's working directory.
+ * @param cwd - the explorer root (absolute).
+ * @param path - an absolute entry path from the fs-tree.
+ * @returns the relative path with '/' separators ('.' for the cwd itself),
+ * or `path` unchanged when it lies outside the cwd.
+ */
+export function relativeTo(cwd: string, path: string): string {
+  const base = cwd.replace(/[\\/]+$/, '')
+  const norm = (value: string): string => value.replace(/\\/g, '/')
+  const nBase = norm(base)
+  const nPath = norm(path)
+  if (nPath === nBase) return '.'
+  if (nPath.startsWith(`${nBase}/`)) return nPath.slice(nBase.length + 1)
+  return path
+}