Podcast USB v0.6.0

- FIX: Already on USB is determined exclusively from files present in the drive
  inventory. Deleted files are no longer counted from stale manifest entries.
  Preparation history stays separate: Previously prepared - not on USB.
  Missing episodes are selectable by default, including previously prepared ones.
- Drive inventory also recognises .MP3 and mixed-case MP3 filename extensions.
- Activity rows have tighter spacing, duration alongside the title and current
  status on the right. Review order and open history panels remain stable.
- Audio preparation now names the current episode and show, with its length,
  action, action percentage, speed and ETA where measurable. Source checks,
  finished-MP3 checks and saving are explicit states; they no longer leave the
  panel saying an old task is Done while work continues. Whole-queue estimates
  and the USB summary remain available. The USB stage still replaces preparation.
- App Settings has its own sidebar page: default PC download folder,
  simultaneous downloads and default date lookback when no successful update
  exists. Existing per-podcast settings remain on the lineup cards. Changing
  the default folder affects future work and does not move existing downloads.
- The first-use lookback defaults to 14 days, configurable from 1 to 365 days.
  After a successful update, its timestamp remains the default starting point.

UPDATE
Close the app, extract and replace its files. Keep runtime and tools folders.
Saved feeds, title rules, filters, pending work and history retain their identity.
