Sync Stats & History window, headless --sync-cli, and Windows remote support (#160)

changes/sync-active-session-frequency-double-count.md

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+type: fixed
+area: sync
+
+- Fixed word/kanji frequencies double-counting across syncs when the remote snapshot contained a stale active session (e.g. after a crash): a word new to the local machine adopted the remote's full lifetime frequency, which already included the active session's partial occurrences, and those occurrences were added again when the session finalized and synced. Newly adopted words/kanji now exclude active-session counts, which arrive once the session completes.

changes/sync-monthly-rollup-timezone.md

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+type: fixed
+area: sync
+
+- Fixed stats sync copying a remote daily rollup that should have been recomputed when the day was the 1st of a month and the machine was on a negative UTC offset (the Americas), which could leave that day's totals wrong after a sync. The rollup day is now read back at local noon instead of UTC midnight, so it always resolves to the correct civil month.

changes/sync-ui-window-close-quit.md

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+type: fixed
+area: sync
+
+- Fixed the app staying alive in the background after closing a standalone Sync window (`subminer sync --ui`) on macOS. The quit re-issued after async will-quit cleanup was dropped by Electron when the cleanup settled within the same tick as the will-quit dispatch; the re-quit now runs on a fresh macrotask, and the standalone Sync window close path uses the same forced-exit fallback as SIGTERM. Windows opened from the tray menu of a running app are unaffected: closing them still leaves the app running.
+- Fixed a sync run hanging when the sync child exited without emitting a terminal result: the run waited on `close`, which a descendant holding the inherited stdio pipes can delay indefinitely, stalling the quit-time sync shutdown. Cancelling now settles the run as soon as the child has exited, and an exit with no result settles after a bounded stdout drain window.
+- Fixed sync progress and error text being corrupted when a multibyte character (e.g. a Japanese media title) straddled a chunk boundary in the sync child's output.

changes/sync-ui-window.md

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+type: added
+area: sync
+
+- Added a sync window (`subminer sync --ui`, or **Sync Stats & History** in the tray menu) for cross-machine immersion sync: saved devices with per-host direction (two-way/push/pull) and remove, one-click sync with live stage-by-stage progress and separate merge summaries for this machine and the remote device, connection testing for first-time setup, cancellable live syncs while the app, stats server, or playback is active, and manual database snapshots (create/merge/reveal/delete, stored in `/tmp/subminer-db-snapshots/` by default). Live sync uses a consistent WAL snapshot and transactional merge, and excludes unfinished sessions until a later sync sees them finalized. Hosts with auto-sync enabled are synced in the background on a configurable interval, including during playback, with results reported as overlay notifications. Hosts synced from the CLI are remembered in `<config dir>/sync-hosts.json` and show up in the window automatically. `subminer sync --ui` launches silently in the background and returns the shell immediately; closing that standalone Sync window shuts down the app.
+- Added `subminer sync <host> --check` to test the SSH connection and remote launcher availability without syncing, using bounded noninteractive probes and settling when the check process exits even if an inherited output pipe remains open, without dropping terminal progress that races process exit. Linux AppImage sync commands clear inherited GUI startup argument transport before launching the bundled engine, then exit directly, so sync window checks and remote snapshots never initialize the GUI or require a display server. Added `subminer sync --json` for machine-readable NDJSON progress output (the protocol the sync window consumes).