fix(sync): run watched sync when a scheduled --sync hits the running instance

The Windows daily-sync task launches `AeroFetch.exe --sync`, but the single-
instance lock routes that relaunch to the `second-instance` handler whenever
AeroFetch is already open. The handler only focused the window and looked for a
link -- it never checked `isSyncLaunch(argv)` -- so the sync never ran. The daily
auto-download of new watched-source uploads therefore did nothing for minimize-
to-tray / always-on users (and the relaunch stole window focus each day).

Add a `runWatchedSync` main->renderer push: on a `--sync` relaunch the handler
now nudges the renderer to run syncWatched() instead of focusing, so the daily
sync fires whether AeroFetch was closed or already running.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/shared/ipc.ts

@@ -105,6 +105,9 @@ export const IpcChannels = {
   scheduledSyncSet: 'sources:scheduled-sync-set',
   /** main → renderer push channel for live indexing progress */
   indexProgress: 'sources:index-progress',
+  /** main → renderer: a scheduled `--sync` relaunch hit the running instance — run the
+   *  watched-source sync here instead of no-opping (Phase J) */
+  runWatchedSync: 'sources:run-sync',
   // --- Built-in yt-dlp terminal (Phase N) ---
   terminalRun: 'terminal:run',
   terminalCancel: 'terminal:cancel',