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
Root cause: renderPreviewPane() was stuffing directory entries into the
viewport model via SetContent(), then rendering through
renderPreviewContent() which wraps the viewport in an additional
border+padding box. This reduced the effective content width by 4
characters compared to the browser pane's innerWidth (width-2), causing
column misalignment where the date column would wrap to the next line.
Fix: For directory previews, directly render entries using the same
renderPaneRows() and renderColumnsHeader() functions as the browser
pane, at the same innerWidth. A temporary BrowserPane is created with
the directory entries, and renderPaneRows() is called with active=false,
producing pixel-identical output to the main file browser.
Removed renderDirectoryPreviewBody() which is no longer needed.
