feat: ADIF monitor — auto-import QSOs from watched external ADIF files

Watches a configurable list of external ADIF files (fldigi RTTY logbook,
N1MM, VarAC…) and imports newly appended QSOs automatically. Each record
goes through the existing UDP-log path, so it gets full enrichment, ±2-minute
dedup (shared udpLogMu, can't race the UDP auto-log) and automatic upload to
the configured external services — no per-file toggles like Log4OM.

A newly added file starts at its current size (offset -1 sentinel → size on
first scan), so the QSOs already in it are NOT bulk-imported; only contacts
logged after it was added come in. Reads only up to the last complete <eor>
so a half-written record waits. Handles truncation/rotation (re-reads from 0,
dedup protects) and persists per-file offsets without clobbering a concurrent
UI edit of the file list.

New Settings → ADIF monitor section: master enable + add/remove/toggle files.
Backend emits adifmon:imported → the grid refreshes and a toast reports the
count. i18n EN + FR.

frontend/src/App.tsx

@@ -1913,7 +1913,16 @@ export default function App() {
         else setError('UDP auto-log: ' + msg);
       }
     });
-    return () => { unsubDX?.(); unsubRC?.(); unsubClear?.(); unsubFlexSpot?.(); unsubProg?.(); unsubLog?.(); };
+    // ADIF monitor imported new QSOs (backend file watcher) → refresh the grid
+    // and show how many, from which file.
+    const unsubAdifMon = EventsOn('adifmon:imported', async (p: any) => {
+      const n = Number(p?.count ?? 0);
+      if (n <= 0) return;
+      await refresh();
+      const file = String(p?.file ?? '').replace(/^.*[\\/]/, '');
+      showToast(`ADIF: ${n} QSO imported${file ? ` from ${file}` : ''}`);
+    });
+    return () => { unsubDX?.(); unsubRC?.(); unsubClear?.(); unsubFlexSpot?.(); unsubProg?.(); unsubLog?.(); unsubAdifMon?.(); };
     // eslint-disable-next-line react-hooks/exhaustive-deps
   }, []);