Fix: create parent directories recursively in check_output_dir

[fuleinist]

Problem

When running hellwal for the first time (or after clearing cache), it fails to write cache files because the parent directories (~/.cache/hellwal/cache/) don't exist yet. The previous implementation only called mkdir() on the final path component, which fails if parent directories are missing.

Solution

Modify check_output_dir to recursively create all parent directories before creating the final directory. This mimics mkdir -p behavior.

Use case

Users running hellwal for the first time will no longer see Cannot write to file and Cannot access directory warnings — the required directories are created automatically.

Testing

• Compiled successfully with make (no warnings)
• Logic: iterates through path components, creating each directory before the next, similar to how mkdir -p works