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/components/SettingsModal.tsx

@@ -909,6 +909,7 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
   const [showBeamMap, setShowBeamMap] = useState(() => localStorage.getItem('opslog.showBeamOnMap') !== '0');
   const [startEqEnd, setStartEqEnd] = useState(() => localStorage.getItem('opslog.startEqualsEnd') === '1');
   const [lookupOnBlur, setLookupOnBlur] = useState(() => localStorage.getItem('opslog.lookupOnBlur') === '1');
+  const [showQsoRate, setShowQsoRate] = useState(() => localStorage.getItem('opslog.showQsoRate') === '1');
   const [catModeBeforeFreq, setCatModeBeforeFreq] = useState(() => localStorage.getItem('opslog.catModeBeforeFreq') === '1');
   // Password-encryption (secret vault) state.
   const [secret, setSecret] = useState<{ has_passphrase: boolean; unlocked: boolean }>({ has_passphrase: false, unlocked: false });
@@ -4194,6 +4195,10 @@ export function SettingsModal({ onClose, onSaved, initialSection, onMainPaneChan
             <Checkbox checked={startEqEnd} onCheckedChange={(c) => { const v = !!c; setStartEqEnd(v); writeUiPref('opslog.startEqualsEnd', v ? '1' : '0'); }} />
             {t('gen.startEqEnd')} <span className="text-xs text-muted-foreground">{t('gen.startEqEndHint')}</span>
           </label>
+          <label className="flex items-center gap-2 text-sm cursor-pointer">
+            <Checkbox checked={showQsoRate} onCheckedChange={(c) => { const v = !!c; setShowQsoRate(v); writeUiPref('opslog.showQsoRate', v ? '1' : '0'); }} />
+            {t('gen.showQsoRate')} <span className="text-xs text-muted-foreground">{t('gen.showQsoRateHint')}</span>
+          </label>
           <label className="flex items-center gap-2 text-sm cursor-pointer">
             <Checkbox checked={lookupOnBlur} onCheckedChange={(c) => { const v = !!c; setLookupOnBlur(v); writeUiPref('opslog.lookupOnBlur', v ? '1' : '0'); }} />
             {t('gen.lookupOnBlur')} <span className="text-xs text-muted-foreground">{t('gen.lookupOnBlurHint')}</span>