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/TROUBLESHOOTING.md

@@ -226,6 +226,22 @@ Check that:
 
 `pdfs sync now` starts a sync pass at once.
 
+### A synced folder stays in error and nothing downloads
+
+The log says `every one of the N synced paths is missing locally; refusing to trash them on
+Drive`. The folder synced before, and now none of its files are on this computer. The sync
+stops so that it does not delete them on Proton Drive too. Check that the folder is the right
+one and that its disk is mounted. To get the files back, choose **Sync From Scratch…** in the
+folder's menu on **Sync → Folders**, or run:
+
+```bash
+pdfs sync list            # find the folder's ID
+pdfs sync reset 1
+```
+
+This downloads what Proton Drive has and uploads what only this computer has. Nothing is deleted.
+If you deleted the files on purpose, delete them on Proton Drive too, or stop syncing the folder.
+
 ### Files named `(sync-conflict …)` appeared
 
 The file changed in two places at once. Nothing was lost. See