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/icons.tsx

@@ -0,0 +1,33 @@
+/**
+ * Icons the sidebar needs beyond the primitives set: a terminal glyph (the
+ * icon library has none) and the per-tab-type icon mapping shared by the
+ * tab strip, the + menu, and the empty-pane cards.
+ */
+import type { ReactNode } from 'react'
+import {
+  IconBranchOutline16, IconCodeOutline16, IconFolderOpen16,
+} from '@deepseek-ai/dsh-client-ui-primitives'
+import type { IconProps } from '@deepseek-ai/dsh-client-ui-primitives'
+import type { TabType } from './state.ts'
+
+/**
+ * Terminal glyph in the app's outline style (1.5px stroke, currentColor):
+ * a rounded frame with a prompt chevron and underscore cursor.
+ */
+export const IconTerminalOutline16 = ({ size = 16, className }: IconProps) => (
+  <svg width={size} height={size} className={className} viewBox="0 0 16 16" fill="none" xmlns="http://www.w3.org/2000/svg">
+    <rect x="1.5" y="2.5" width="13" height="11" rx="2" stroke="currentColor" strokeWidth="1.5" />
+    <path d="M4.5 6.25 6.75 8 4.5 9.75" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" strokeLinejoin="round" />
+    <path d="M8.5 10.4h3" stroke="currentColor" strokeWidth="1.5" strokeLinecap="round" />
+  </svg>
+)
+
+/** The one icon per tab type (shared by tabs, the + menu, and pane cards). */
+export function tabTypeIcon(type: TabType, size = 16): ReactNode {
+  switch (type) {
+    case 'explorer': return <IconFolderOpen16 size={size} />
+    case 'git': return <IconBranchOutline16 size={size} />
+    case 'terminal': return <IconTerminalOutline16 size={size} />
+    case 'editor': return <IconCodeOutline16 size={size} />
+  }
+}