Fallback source file counting for non-git directories

[Taiki92777]

Summary

Source file count can show 0 in non-git directories because counting relies only on git ls-files.

Current behavior

When scanning a React project that is not inside a Git repository, source file detection fails and count becomes 0.

Reproduction

1. Create a React project directory with .tsx files.
2. Do not initialize Git.
3. Run react-doctor ..

Observed:

• output says Found 0 source files.

Expected behavior

If git-based counting is unavailable, fallback to filesystem traversal with sensible ignores.

[Taiki92777]

Proposed fix is in PR #54: #54

[aidenybai]

Verified fixed: countSourceFilesViaFilesystem() fallback exists, countSourceFiles() tries git first then falls back via ??.