(tui): Decode percent-escaped bare local file links (#16810)

Addresses #16622

Problem: bare local file links in TUI markdown render percent-encoded
path bytes literally, unlike file:// links.

Solution: decode bare path targets before local-path expansion and add
regression coverage for spaces and Unicode.

codex-rs/Cargo.lock

@@ -2854,6 +2854,7 @@ dependencies = [
  "unicode-segmentation",
  "unicode-width 0.2.1",
  "url",
+ "urlencoding",
  "uuid",
  "vt100",
  "webbrowser",