Add a Files section to room info listing all sent attachments (#33)

Lists files and images actually attached to sent messages in a room,
newest first, with click-through to a lightbox, preview modal, or direct
download depending on type. Queries through messages.image_id/file_id
so an upload that was never sent doesn't show up as a phantom entry.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

frontend/src/lib/fileSize.ts

@@ -0,0 +1,5 @@
+export function formatFileSize(bytes: number): string {
+  if (bytes < 1024) return `${bytes} B`
+  if (bytes < 1024 * 1024) return `${(bytes / 1024).toFixed(1)} KB`
+  return `${(bytes / (1024 * 1024)).toFixed(1)} MB`
+}