fix: getFileCategory must strip the directory before matching basename entries

[chuenchen309]

Problem

getFileCategory extracts the extension with filePath.split(".").pop() on the full path:

export function getFileCategory(filePath: string): FileCategory {
  const ext = filePath.split(".").pop()?.toLowerCase() ?? ""
  return EXT_MAP[ext] ?? "unknown"
}

EXT_MAP has two extensionless basename entries — dockerfile: "code" and makefile: "code" — but a path-prefixed file with no . returns the whole path as the "extension", so they never match. Every caller passes a full path (preview.path, selectedFile, …):

getFileCategory("Dockerfile")            -> "code"     (bare name works)
getFileCategory("/project/Dockerfile")   -> "unknown"  (bug)
getFileCategory("/a/b/Makefile")         -> "unknown"  (bug)

unknown is in isBinary()'s list, so a Dockerfile/Makefile is treated as a non-previewable binary instead of previewable code.

Fix

Mirror the sibling getFileExtension() (which already strips the directory first), then fall back to the basename itself when it has no dot, so the two basename entries resolve.

Test

Added a case to file-types.test.ts covering bare and path-prefixed Dockerfile/Makefile; it fails before the fix and passes after. Existing tests unchanged.

Prepared with AI assistance and reviewed before submission.