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.

crates/pdfs-core/src/control.rs

@@ -480,6 +480,11 @@ pub enum Request {
     /// Pause or resume one mirror folder's reconcile, independent of the
     /// global pause. Persisted until resumed. Replies with [`Response::Ok`].
     SetSyncFolderPaused { id: i64, paused: bool },
+    /// Forget what a mirror folder synced before, so its next pass compares
+    /// both sides afresh: what is on one side only is copied to the other, and
+    /// a file that differs keeps both versions. Nothing is deleted. Replies
+    /// with [`Response::Ok`].
+    ResetSyncFolder { id: i64 },
     /// Force a reconcile pass: one folder by id, or all when `id` is `None`.
     /// Replies with [`Response::Ok`].
     SyncNow { id: Option<i64> },
@@ -3209,6 +3214,7 @@ mod tests {
                 id: 3,
                 paused: true,
             },
+            Request::ResetSyncFolder { id: 3 },
             Request::SetBandwidthLimits {
                 upload: 1_000_000,
                 download: 0,