fix: sanitize filenames for cross-platform compatibility

[flexiondotorg]

• Add sanitize_filename() function to utils.rs that replaces invalid filesystem characters (< > : " | ? * and control characters) with underscores
• Handle Windows reserved names (CON, PRN, AUX, NUL, COM1-9, LPT1-9) by inserting underscore before extension
• Preserve directory structure in filenames with path separators
• Trim leading/trailing spaces and trailing dots from path components
• Apply sanitization in main.rs before downloading files
• Display warnings when filenames are modified with before/after
• Show summary count of sanitized files
• Add comprehensive unit tests covering edge cases

Fixes #63

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Summary by cubic

Sanitizes filenames before download to ensure cross-platform compatibility and prevent filesystem errors (fixes #63).

• Bug Fixes
  • Replace invalid characters (< > : " | ? * \ and control chars) with underscores.
  • Handle Windows reserved names (CON, PRN, AUX, NUL, COM1–9, LPT1–9) by adding an underscore before the extension.
  • Trim leading/trailing spaces and trailing dots in each path part; preserve path separators.
  • Apply sanitization in main.rs before downloads; warn per file and show a total count.
  • Add unit tests covering edge cases.

^{Written for commit b476e8a. Summary will update on new commits.}

[cubic-dev-ai]

No issues found across 2 files