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/CLI.md

@@ -84,6 +84,7 @@ commands are for scripts and for cases where the mount is not available.
 | `pdfs sync rm ID [--delete-remote]` | Stop syncing. Local files stay; `--delete-remote` also removes the copy on Proton Drive. |
 | `pdfs sync mode ID mirror\|ondemand` | Switch between a full local copy (`mirror`) and online only (`ondemand`). |
 | `pdfs sync now [ID]` | Start a sync pass now, for all folders or one. |
+| `pdfs sync reset ID` | Forget what a synced folder synced before and compare it with Proton Drive again. A file on one side only is copied to the other, and a file that differs keeps both versions. Nothing is deleted. |
 | `pdfs sync pause [--for 1h] [--folder ID]` | Pause uploads and synced folders. Reading files keeps working. `--for` ends the pause by itself (`30m`, `1h`, `2d`). |
 | `pdfs sync resume [--folder ID]` | Resume after a pause. |
 | `pdfs sync limit [--up RATE] [--down RATE]` | Cap bandwidth, for example `500K` or `2M` per second. `0` removes a cap. Without options, prints the current caps. |