fix(explorer): pin reference affordance to row tail

tests/file-tree-reference-tail.spec.ts

@@ -0,0 +1,17 @@
+/**
+ * The @-reference button and its transient copied label share a file-tree
+ * row with a shrinking filename. Both need the row's only auto margin so a
+ * short filename cannot leave the affordance stranded in the middle.
+ */
+import { readFileSync } from 'node:fs'
+import { describe, expect, it } from 'vitest'
+
+const css = readFileSync('src/client/sidebar.module.css', 'utf8')
+
+describe('FileTree @-reference row tail', () => {
+  it('anchors both mutually-exclusive reference affordances after a short filename', () => {
+    const rule = css.match(/\.explorerRef,\s*\.explorerCopied\s*\{([\s\S]*?)\n\}/)?.[1]
+    expect(rule).toBeDefined()
+    expect(rule).toMatch(/margin-left:\s*auto/)
+  })
+})