feat: QSO rate meter (10/60 min) in the header

Opt-in via Settings→General (portable pref opslog.showQsoRate). Shows the
contest-style QSO rate in QSOs/hour, projected from the trailing 10-minute
(count ×6) and 60-minute windows, between the widget icons and propagation.

Backend: qso.RecentRate counts QSOs whose start time falls in each trailing
window, scanning only the last 400 rows (cheap on a large log); App.GetQSORate
exposes the 10/60-min counts. Frontend refreshes on qso:logged and a 30s tick.

The meter shares the propagation grid cell — the header is a fixed 6-column
grid, so adding it as its own child pushed profile/band-map/compact onto a
second row. i18n EN + FR.

frontend/src/lib/uiPref.ts

@@ -19,6 +19,7 @@ const PORTABLE_KEYS = [
   'opslog.showRotor',     // rotor compass shown next to the keyers
   'opslog.showBeamOnMap', // antenna beam lobe drawn on the Main map
   'opslog.startEqualsEnd',// log TIME_ON = TIME_OFF (QSO time = completion time)
+  'opslog.showQsoRate',   // QSO-rate meter (10/60 min) shown in the header
   'opslog.catModeBeforeFreq', // send CAT mode before frequency (older rigs)
   'opslog.bandMapBands',  // bands shown side-by-side in the Band Map tab
   'opslog.mapAutoZoomDX', // Main map: auto-zoom to the DX (vs free pan/zoom)