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/components/QSOEditModal.tsx

@@ -412,9 +412,9 @@ export function QSOEditModal({ qso, onSave, onDelete, onClose, countries = [], b
                     value={draft.callsign ?? ''} onChange={(e) => set('callsign', e.target.value)} />
                 </div>
                 <div className="flex flex-col w-20"><Label>S</Label>
-                  <Combobox value={draft.rst_sent ?? ''} options={rstOptions(draft.mode ?? '', rstLists)} allowFreeText commitOnType onChange={(v) => set('rst_sent', v)} /></div>
+                  <Combobox value={draft.rst_sent ?? ''} options={rstOptions(draft.mode ?? '', rstLists)} commitOnType onChange={(v) => set('rst_sent', v)} /></div>
                 <div className="flex flex-col w-20"><Label>R</Label>
-                  <Combobox value={draft.rst_rcvd ?? ''} options={rstOptions(draft.mode ?? '', rstLists)} allowFreeText commitOnType onChange={(v) => set('rst_rcvd', v)} /></div>
+                  <Combobox value={draft.rst_rcvd ?? ''} options={rstOptions(draft.mode ?? '', rstLists)} commitOnType onChange={(v) => set('rst_rcvd', v)} /></div>
                 <Button type="button" variant="outline" className="h-10" onClick={fetchLookup} disabled={looking}
                   title={t('qedit.fetchTitle')}>
                   {looking ? <Loader2 className="size-4 animate-spin" /> : <Search className="size-4" />} {t('qedit.fetch')}