feat: name byte-path file downloads via Content-Disposition

[william0wang]

Summary

• Remote file endpoint: byte-path responses (HEAD + GET) now send a
  Content-Disposition: inline header carrying the real filename.
• The URL basename is always "file" (the name lives in a query param), so
  browser direct-downloads previously saved nameless.
• Non-ASCII names survive via the RFC 6266/5987 filename* form; the ASCII
  fallback degrades to underscores. Quoting/CR/LF are sanitized (no header
  injection).
• Line windows (viewer-internal partial views) intentionally stay unnamed.

Test plan

• New case in tests/remote-file-endpoint.test.ts: ASCII name, non-ASCII
  (中文) name, and line-window exclusion; requests go through the hub
  proxy, proving the header survives proxying.
• Full suite 690/690, tsc --noEmit, prettier clean.

Release 0.11.6.