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.

app.go

@@ -483,6 +483,7 @@ type App struct {
 	dvkPttKeyed    bool              // we keyed PTT for a voice message; unkey when it ends
 	pttMu          sync.Mutex
 	udpLogMu       sync.Mutex      // serialises UDP auto-log so concurrent packets can't both pass the dedup check
+	adifMonMu      sync.Mutex      // guards the ADIF-monitor config (file list + per-file read offsets)
 	pttPort        serial.Port     // open serial port while PTT (RTS/DTR) is asserted
 	pttKeyedMethod string          // "cat" | "rts" | "dtr" while keyed; "" when idle
 	pttGen         int64           // bumped on every key; a delayed unkey only fires if unchanged (guards against a stale release cutting a new transmission)
@@ -808,6 +809,7 @@ func (a *App) startup(ctx context.Context) {
 	a.logDb = logbookConn
 	a.qso = qso.NewRepo(logbookConn)
 	go a.rebuildWorkedIndex() // in-memory worked-index for per-spot alert checks
+	go a.adifMonitorLoop()    // watch external ADIF files (fldigi, N1MM…) for new QSOs
 
 	// cty.dat for offline DXCC / country resolution. Cached on disk; first
 	// run downloads it from country-files.com in the background so startup