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

tests/unit.spec.ts

@@ -7,6 +7,7 @@ import {
   type SidebarState, type SplitNode,
 } from '../src/client/state.ts'
 import { extOf, languageKeyForExt } from '../src/client/lang.ts'
+import { relativeTo } from '../src/client/paths.ts'
 import { producedForClosing, resolveSidebarPath, selectProducedFiles } from '../src/client/produced-files.ts'
 import { defaultShell, ensureSpawnHelper } from '../src/pty-manager.ts'
 
@@ -318,6 +319,25 @@ describe('persisted state sanitization', () => {
   })
 })
 
+describe('path helpers', () => {
+  it('derives relative paths under the cwd (and "." for the cwd itself)', () => {
+    expect(relativeTo('/Users/me/code', '/Users/me/code/src/main.ts')).toBe('src/main.ts')
+    expect(relativeTo('/Users/me/code', '/Users/me/code')).toBe('.')
+    expect(relativeTo('/Users/me/code/', '/Users/me/code/src/a/b.ts')).toBe('src/a/b.ts')
+  })
+
+  it('falls back to the path unchanged when it lies outside the cwd', () => {
+    expect(relativeTo('/Users/me/code', '/Users/other/x.ts')).toBe('/Users/other/x.ts')
+    expect(relativeTo('/Users/me/code', '/Users/me/codex/y.ts')).toBe('/Users/me/codex/y.ts')
+  })
+
+  it('handles windows roots and mixed separators', () => {
+    expect(relativeTo('C:\\Users\\me', 'C:\\Users\\me\\src\\a.ts')).toBe('src/a.ts')
+    expect(relativeTo('C:\\Users\\me', 'C:/Users/me/src/a.ts')).toBe('src/a.ts')
+    expect(relativeTo('C:\\Users\\me\\', 'C:\\Users\\me')).toBe('.')
+  })
+})
+
 describe('editor language mapping', () => {
   it('derives extensions from paths', () => {
     expect(extOf('/a/b/main.tsx')).toBe('tsx')