feat(fallbacks): add per-rule dest override

Operators can now type an explicit dest (e.g. "8443", "127.0.0.1:8443",
"/dev/shm/x.sock") on each fallback row to override the auto-resolved
child listen+port. Empty keeps the existing auto behavior.

Adds the column to inbound_fallbacks (GORM AutoMigrate), threads it
through the panel form, API docs, and translations.

frontend/src/pages/inbounds/InboundFormModal.tsx

@@ -365,13 +365,21 @@ export default function InboundFormModal({
       return;
     }
     setFallbacks(
-      (msg.obj as { childId: number; name?: string; alpn?: string; path?: string; xver?: number }[])
+      (msg.obj as {
+        childId: number;
+        name?: string;
+        alpn?: string;
+        path?: string;
+        dest?: string;
+        xver?: number;
+      }[])
         .map((r) => ({
           rowKey: `fb-${++fallbackKeyRef.current}`,
           childId: r.childId,
           name: r.name || '',
           alpn: r.alpn || '',
           path: r.path || '',
+          dest: r.dest || '',
           xver: r.xver || 0,
         })),
     );
@@ -385,6 +393,7 @@ export default function InboundFormModal({
         name: c.name,
         alpn: c.alpn,
         path: c.path,
+        dest: c.dest,
         xver: Number(c.xver) || 0,
         sortOrder: i,
       })),
@@ -437,6 +446,7 @@ export default function InboundFormModal({
       name: '',
       alpn: '',
       path: '',
+      dest: '',
       xver: 0,
     }]);
   };
@@ -445,11 +455,11 @@ export default function InboundFormModal({
     setFallbacks((prev) => prev.map((r) => {
       if (r.rowKey !== rowKey) return r;
       // When the picker selects a new child inbound and the row hasn't
-      // been hand-edited yet (sni/alpn/path all blank, xver = 0), pull
-      // the SNI/ALPN/Path defaults off that child. Operators who
+      // been hand-edited yet (sni/alpn/path/dest all blank, xver = 0),
+      // pull the SNI/ALPN/Path defaults off that child. Operators who
       // intentionally typed values keep them — we only fill the empties.
       if (typeof patch.childId === 'number' && patch.childId !== r.childId) {
-        const isPristine = !r.name && !r.alpn && !r.path && r.xver === 0;
+        const isPristine = !r.name && !r.alpn && !r.path && !r.dest && r.xver === 0;
         if (isPristine) return { ...r, ...patch, ...deriveFallbackDefaults(patch.childId) };
       }
       return { ...r, ...patch };
@@ -490,6 +500,7 @@ export default function InboundFormModal({
             name: derived.name ?? '',
             alpn: derived.alpn ?? '',
             path: derived.path ?? '',
+            dest: '',
             xver: derived.xver ?? 0,
           };
         });
@@ -1079,6 +1090,12 @@ export default function InboundFormModal({
               value={record.path}
               onChange={(e) => updateFallback(record.rowKey, { path: e.target.value })}
             />
+            <InputAddon>Dest</InputAddon>
+            <Input
+              placeholder={t('pages.inbounds.fallbacks.destPlaceholder') || 'auto'}
+              value={record.dest}
+              onChange={(e) => updateFallback(record.rowKey, { dest: e.target.value })}
+            />
             <InputAddon>xver</InputAddon>
             <InputNumber
               min={0}