fix: restrict RST fields to valid report values

The RST comboboxes allowed free text, so a report typed by hand that isn't in
the mode's list (e.g. "600") was committed. Drop allowFreeText on the four RST
fields (QSO entry + edit modal) and make commit-on-type push a value only when
it's actually in the list; a leftover invalid entry reverts to the last valid
value on blur. Programmatically filled values (FT8 SNR over UDP, presets) are
unaffected — only manual typing is constrained.

frontend/src/App.tsx

@@ -2879,12 +2879,12 @@ export default function App() {
   );
   const rstTxBlock = (
     <div className="flex flex-col w-20"><Label className="mb-1 h-3.5">{t('field.rstTx')}</Label>
-      <Combobox value={rstSent} options={rstOptions(mode, rstLists)} allowFreeText commitOnType onChange={(v) => { setRstSent(v); rstUserEditedRef.current = true; }} />
+      <Combobox value={rstSent} options={rstOptions(mode, rstLists)} commitOnType onChange={(v) => { setRstSent(v); rstUserEditedRef.current = true; }} />
     </div>
   );
   const rstRxBlock = (
     <div className="flex flex-col w-20"><Label className="mb-1 h-3.5">{t('field.rstRx')}</Label>
-      <Combobox value={rstRcvd} options={rstOptions(mode, rstLists)} allowFreeText commitOnType onChange={(v) => { setRstRcvd(v); rstUserEditedRef.current = true; }} />
+      <Combobox value={rstRcvd} options={rstOptions(mode, rstLists)} commitOnType onChange={(v) => { setRstRcvd(v); rstUserEditedRef.current = true; }} />
     </div>
   );
   // DX country flag, shown large next to RST (moved here from the Country field).