fix(sync,gui,cli): get a synced folder unstuck when its local copy is gone (#46)

A mirror folder whose files were all gone from this computer stayed in
error for good. The wipe guard rightly refuses to trash the whole folder
on Drive, but nothing got the folder out of that state: a queued switch
to on-demand waits for a pass that ends idle, and its push pass hit the
same guard. Only removing the folder from sync cleared the baseline that
made the empty folder read as "delete everything".

- The push pass for a switch to on-demand forgets the baseline rows of a
  local copy that is gone in full instead of trashing them, so the
  switch goes ahead and the mount shows what Drive has.
- New Request::ResetSyncFolder, `pdfs sync reset ID` and "Sync From
  Scratch…" in a synced folder's menu clear a mirror folder's baseline
  (and a queued switch to on-demand, whose push pass would upload a
  duplicate of every file). The next pass compares both sides as on the
  first sync: one-sided files are copied across, files that differ keep
  both versions, nothing is deleted.
- The wipe guards' messages name the reset; restore logs why it skipped
  a folder.

Two simulations cover it. The on-demand one failed in exactly the
reported state (mirror, pending ondemand, error) before the fix.
All 8 catalogs translated.

docs/SYNCED_FOLDERS.md

@@ -76,6 +76,14 @@ again when they move into a synced one.
 Proton Drive. To delete the copy on Proton Drive as well, tick **Also delete from Proton Drive**,
 or add `--delete-remote`.
 
+## Sync a folder from scratch
+
+**Sync From Scratch…** in the folder's menu, or `pdfs sync reset ID`, makes this computer forget
+what it synced in the folder before and compare it with Proton Drive again, as on the first sync.
+A file on one side only is copied to the other, and a file that differs keeps both versions.
+Nothing is deleted. Use it when a folder's files went missing on this computer and the sync
+refuses to go on (see [Troubleshooting](TROUBLESHOOTING.md#a-synced-folder-stays-in-error-and-nothing-downloads)).
+
 ## Excluding files with `.pdfsignore`
 
 Synced folders skip build output, dependency folders and editor leftovers. Two sets of rules