[xiaofeilovecoding]

Fixes #2006.

Resolve relative Markdown images from the document currently open in a folder or bundle session. Reuse the existing cross-platform directory helper, while preserving the session base for single-file content.

Validation:

• Original implementation: the two new nested-document cases return HTTP 404 from the real image reader; the single-file/remote-image case passes.
• Fixed implementation: 39 related tests pass, including all seven bundle tests. The regressions navigate the file tree and read the actual rendered image URLs, checking distinct PNG bytes after switching directories.
• A focused editor typecheck reports the same 24 diagnostics on the original and fixed source. Full workspace typecheck, Ubuntu CI/build and real-host browser testing were not run locally.