Fixes out-of-sync preview thumbnails & clearer config warnings

Fixes

• Preview thumbnails could drift out of sync when scrubbing in Plex.
  If you'd ever changed your thumbnail interval (e.g. 2s → 5s), leftover frames
  from the earlier run were still being counted into the new preview strip. That
  made the strip longer than the movie — a 1h55m film could end up with 5h45m of
  previews — so the thumbnails lagged behind the playhead. Frames are now cleared
  before each run. Re-generate any affected files to fix them.

• A read-only or wrongly-owned /config no longer looks like a frozen app.
  It used to surface as an opaque SQLite "readonly database" error while the UI
  sat there doing nothing. You now get one clear banner naming the actual fix —
  ownership, a :ro mount, or a full disk.

• No more bogus "network filesystem" warning on Unraid and mergerfs.
  The config check treated every FUSE mount as a network share. Unraid user
  shares and mergerfs are local storage, so that warning was simply wrong. Unraid
  users now get an accurate note (with the /mnt/cache/appdata tip), and these
  advisories can be dismissed for good.