fix(fallbacks): allow free-form dest entries for external servers (#4748)

Since v3.1.0 every fallback row had to reference a panel inbound via childId, so rows with only a free-form dest (e.g. 8080 or 127.0.0.1:8080 to an external Nginx) were silently dropped at three layers: the frontend save filter, the backend SetByMaster guard, and BuildFallbacksJSON. A row is now valid when it has a child OR an explicit dest; self-references normalize to childId 0, and BuildFallbacksJSON prefers an explicit dest (also fixing rows whose child was deleted). UI gains allowClear on the child picker; help text updated across all locales. Verified end-to-end in Docker: a free-form dest fallback now persists and is injected into the live xray config. Refs #4554, #4639.

frontend/src/pages/inbounds/form/FallbacksCard.tsx

@@ -44,12 +44,13 @@ export default function FallbacksCard({
               value={record.childId}
               options={fallbackChildOptions}
               placeholder={t('pages.inbounds.fallbacks.pickInbound') || 'Pick an inbound'}
+              allowClear
               showSearch={{
                 filterOption: (input, option) =>
                   ((option?.label as string) || '').toLowerCase().includes(input.toLowerCase()),
               }}
               style={{ width: '100%' }}
-              onChange={(v) => updateFallback(record.rowKey, { childId: v })}
+              onChange={(v) => updateFallback(record.rowKey, { childId: v ?? null })}
             />
             <Button
               disabled={idx === 0}