desktop: syncs on its own — at launch, soon after an edit, every few minutes and on focus

Until now the only caller of the sync engine was the "Sync now" button. A worker
thread now owns every cycle (the button's included, so two never overlap):

- launch: one cycle as the app opens;
- edit: every 10s it reads a fingerprint of the pending set and sends when that
  moved. A fingerprint rather than "anything pending", because a rejected change
  stays pending and would otherwise be resent every tick forever;
- timer: a pull every 5 minutes with nothing to send;
- focus: at most once per 30s.

Failed automatic cycles back off (doubling from 10s to 5 minutes). A panicking
cycle counts as a failed one rather than ending the thread. Every cycle is
emitted as inkwell://synced: the board reloads when the pull changed something,
and the Sync screen shows the last automatic failure. No final push on quit;
the launch cycle sends whatever was left.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/src/sync/engine.rs

@@ -13,7 +13,7 @@ use super::push;
 use super::state;
 use crate::local::Db;
 
-#[derive(Debug, Serialize)]
+#[derive(Debug, Clone, Serialize)]
 pub struct SyncOutcome {
     pub push: push::PushSummary,
     pub pull: pull::PullSummary,