fix: harden proxy auth and SSRF defenses

lib/auth/active-account-slot.ts

@@ -0,0 +1,18 @@
+import { useAccountStore } from '@/stores/account-store';
+import { useAuthStore } from '@/stores/auth-store';
+
+export function getActiveAccountSlot(): number | null {
+  const authState = useAuthStore.getState();
+  const accountState = useAccountStore.getState();
+  const activeAccountId = authState.activeAccountId ?? accountState.activeAccountId;
+  const activeAccount = activeAccountId
+    ? accountState.getAccountById(activeAccountId)
+    : accountState.getActiveAccount();
+
+  return typeof activeAccount?.cookieSlot === 'number' ? activeAccount.cookieSlot : null;
+}
+
+export function getActiveAccountSlotHeaders(): Record<string, string> {
+  const slot = getActiveAccountSlot();
+  return slot === null ? {} : { 'X-JMAP-Cookie-Slot': String(slot) };
+}