Rail: proper folder icon for Files (inline SVG, matches the monochrome glyphs)

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

web/src/Rail.tsx

@@ -1,10 +1,10 @@
-import { useState } from 'react'
+import { ReactNode, useState } from 'react'
 import { Term } from './api'
 import { Spinner } from './ui'
 
 // A group is either a page (clicking the head opens it — Dashboard) or a folder of
 // leaves (Terminal). `soon` groups are inert placeholders for the roadmap.
-export interface Group { id: string; label: string; icon: string; soon?: boolean; page?: boolean }
+export interface Group { id: string; label: string; icon: ReactNode; soon?: boolean; page?: boolean }
 
 interface Props {
   groups: Group[]; terms: Term[]; active: string | null; view: string; creating?: boolean