feat: Add Unicode support for filenames

[rawwerks]

Summary

Previously, the handelize function only accepted filenames containing ASCII alphanumeric characters [a-zA-Z0-9]. This caused indexing to fail for files with non-ASCII names.

Before: Files like 腾讯混元.md, 日本語.md, café-notes.md would throw an error during indexing.

After: Unicode letters and numbers are now supported in filenames across all languages.

Changes

• Updated validation regex to use Unicode property escapes [\p{L}\p{N}] which match any Unicode letter or number
• Updated character replacement regex to preserve Unicode letters/numbers instead of stripping them
• Updated tests to verify Unicode filename support

Test Plan

• All existing handelize tests pass
• New tests added for Chinese, Japanese, Korean filenames
• Mixed Unicode/ASCII filenames work correctly
• Folder paths with Unicode work correctly

Example

// Before: throws "no valid filename content"
handelize("腾讯混元.md") 

// After: returns "腾讯混元.md"
handelize("腾讯混元.md") // => "腾讯混元.md"
handelize("日本語-notes.md") // => "日本語-notes.md"  
handelize("café-notes.md") // => "café-notes.md"

🤖 Generated with Claude Code

[neoromantic]

@tobi appreciate if you could merge this one!