fix: digital-mode grouping ignored by the spots already listed

The backend honours the setting: it normalises the log side and the spot's own
mode through the same grouper, so an FT4 spot on a band worked in FT8 reads as
worked. That was not the fault.

Spot statuses are cached per call+band+mode and the cache never expired. Ticking
the box in Settings therefore changed nothing on screen — every spot already
listed kept the answer to the old question, and only new arrivals were judged by
the new rule. The cache is now dropped when settings are saved.

Also: writeUiPref swallowed a failed database write. The interface kept working
from localStorage while the BACKEND — which reads some of these keys, this one
among them — still saw the old value. The two disagreeing in silence is exactly
the shape of this bug, so the failure is now logged.

frontend/src/App.tsx

@@ -6318,7 +6318,14 @@ export default function App() {
         <SettingsModal
           initialSection={settingsSection}
           onClose={() => { setShowSettings(false); setSettingsSection(undefined); }}
-          onSaved={() => { loadStation(); loadLists(); loadCATCfg(); reloadWk(); refreshManualRecReady(); }}
+          onSaved={() => {
+            loadStation(); loadLists(); loadCATCfg(); reloadWk(); refreshManualRecReady();
+            // Drop the cached spot statuses. They are computed once per
+            // call+band+mode and never expire, so a rule change in Settings —
+            // grouping the digital modes into one slot, above all — left every
+            // spot already on screen showing the answer to the OLD question.
+            setSpotStatus({});
+          }}
           onMainPaneChanged={(side, v) => { if (side === 'left') setMainPaneLeft(v as MainPaneKind); else setMainPaneRight(v as MainPaneKind); }}
           flexAvailable={catState.backend === 'flex'}
           icomAvailable={catState.backend === 'icom'}