Fix local Markdown image uploads for absolute and home paths

[wosheesh]

Markdown images such as ![animation](/absolute/path/animation.gif) lose their leading slash before Api.get_image() checks the filesystem. The path is then treated as relative to the server's working directory. Upload exceptions are swallowed, leaving a broken local path in the saved draft.

Preserve existing absolute paths, expand ~, and decode Markdown-escaped filenames before upload. Keep the legacy leading-slash fallback only when the absolute file is absent and the corresponding relative file exists. Missing files, upload failures, and invalid upload responses now raise an error naming the file. HTTP(S) and protocol-relative URLs remain unchanged, as do sources rendered without an API client. PNG, JPEG, GIF, and WebP continue through the existing upload implementation without conversion.

Validation:

• 25 offline regression cases with mocked Api.get_image, including absolute paths for all four formats, home paths, spaces/Unicode/percent signs, missing files, directories, absolute-vs-relative precedence, upload failures, invalid responses, remote URLs, and rendering without an API.
• Full offline suite on this branch: python -m pytest -q -m "not live" --strict-markers — 225 passed, 8 live tests deselected.
• Live MCP round trip using the same renderer: created a new disposable draft with an absolute animated GIF path and publish=False, prepublish=False, send=False; re-fetched it and verified the stored https://substack-post-media.s3.amazonaws.com/...gif source; deleted only the disposable draft. Nothing published and no email sent.
• Pre-commit passes on all changed files. Running it repository-wide also detects pre-existing formatting issues in tests/substack/test_widget_preservation.py; that unrelated file is intentionally excluded from this PR.

This branch contains only the image fix, its regression tests, and documentation/changelog updates. It does not include fork-specific MCP editing or throttling changes.

[ma2za]

Request changes: the new local-image test module is not portable to Windows. On Windows, 23 tests pass and two fail: est_home_path_expanded sets HOME, but pathlib.expanduser() uses USERPROFILE; and est_absolute_file_takes_priority_over_relative slices a drive-qualified absolute path with [1:], producing an invalid :\... path. Please make both fixtures platform-aware and run the suite on Windows (or add Windows CI) before approval. The implementation itself did successfully upload Windows absolute paths in my review.

[ma2za]

Follow-up review: fixed Windows portability in the new local-image tests. The updated branch passes 24 focused tests with one platform-specific skip and 224 offline tests overall. Production image-path handling was verified with a real Windows absolute path upload.