Add rich preview support for three new file categories by leveraging
external CLI tools with graceful fallback when tools are missing.
- PDF: text extraction via pdftotext, page count via pdfinfo
- Audio: metadata via ffprobe (duration, bitrate, codec, sample rate, channels)
- Video: metadata via ffprobe (duration, bitrate, video/audio codec, resolution)
- New PreviewKind constants: PDF, Audio, Video
- New Metadata fields for extended preview data
- New extension maps and Category() entries for pdf/audio/video
- Icons: PDF (), audio (), video () in preview header
Closes#5
