feat: add archive index page generation with comprehensive metadata

Add --create-archive flag to generate organized index pages linking all downloaded posts.

Features:
- Archive pages in HTML/Markdown/Text formats matching post format
- Post metadata: titles, publication/download dates, descriptions, cover images
- Automatic sorting by publication date (newest first)
- Enhanced post extraction for subtitle (.subtitle) and cover image (og:image)
- Integration with single post and bulk download workflows
- Comprehensive test coverage (30+ new test cases)
- Complete documentation and technical specifications

Usage: sbstck-dl download --url https://example.substack.com --create-archive

Generated files: index.{html|md|txt} in output directory root

chore: bump version to v0.6.7

chore: bump version to v0.6.6

fix: prevent Windows test timeout in file download test server

The test server's timeout simulation was using an infinite select{} which
caused the Windows tests to hang indefinitely. Added a 5-second timeout
to ensure the server responds with a proper timeout status instead of
blocking forever.

chore: bump version to v0.6.4

chore: bump version to v0.6.3

fix: prevent comma-separated URL fragments in srcset parsing

Refactors srcset parsing to handle Substack CDN URLs that contain commas
in their parameters (like w_424,c_limit,f_auto,q_auto:good). Previous
naive comma splitting was treating URL parameter fragments as separate
entries, causing malformed image paths in downloaded HTML.

- Adds robust regex-based parseSrcsetEntries() for URLs with embedded commas
- Updates extractURLFromSrcset, extractAllURLsFromSrcset, and updateSrcsetAttribute
- Updates test cases to use proper HTTP URLs instead of filenames
- Verified fix works on real Substack posts with complex CDN URLs

Fixes regression from commit 57657b2 where tests were failing due to
stricter URL validation conflicting with test expectations.

chore: bump version to v0.6.1

chore: bump version to v0.6.0

fix: resolve Windows CI test failures for image downloader

Remove backslash check in generateSafeFilename to prevent false matches on Windows paths
Use filepath.Join in test to ensure cross-platform path compatibility