test: cover supported file-type detection helpers

[McanKul]

Goal

Add focused unit coverage for the file-type helpers in src/lib/fileTypes.ts so changes to accepted import formats cannot silently break drag-and-drop or file-picker behavior.

Scope

• Add src/lib/fileTypes.test.ts.
• Exercise isImagePath, isPdfPath, isOfficePath and isSupportedPath directly.
• Keep the tests pure; no Tauri runtime or fixture files are required.

Acceptance criteria

• Image coverage includes PNG, JPG/JPEG, GIF, BMP, WebP, TIF/TIFF and HEIC/HEIF.
• Office coverage includes DOC/DOCX, XLS/XLSX, PPT/PPTX, ODT/ODS/ODP, RTF, CSV and HTM/HTML.
• PDF, image and Office extensions are accepted with mixed-case filenames and both POSIX and Windows-style paths.
• isSupportedPath is verified as the union of the three supported groups.
• Missing extensions, unsupported extensions and near misses such as report.pdf.exe, photo.heic.tmp, SVG and DOCM are rejected.
• Production behavior stays unchanged unless a test exposes a documented defect.

Verification

npm test -- src/lib/fileTypes.test.ts
npm run typecheck