feat(codex): add unified session history toggle for official providers

Codex buckets resume history by the model_provider id recorded in each
session: official runs (no key, built-in "openai") and cc-switch
third-party runs (shared "custom") are mutually invisible in the resume
picker. Add an opt-in setting that runs official providers under the
shared "custom" id so future official sessions land in the same history
bucket as third-party ones. Forward-only by design: existing sessions
are not migrated.

When enabled, official live config.toml gets model_provider = "custom"
plus a [model_providers.custom] entry that mirrors the built-in openai
provider (requires_openai_auth routes auth to the ChatGPT login in
auth.json, name "OpenAI" keeps is_openai() feature gates, explicit
supports_websockets/wire_api restore built-in defaults). auth.json is
untouched.

Key invariants:
- Injection lives only in the live config: switch-away backfill strips
  the exact injected shape, so stored provider configs stay clean and
  turning the toggle off fully reverts on the next write.
- Toggle changes apply immediately via a takeover-aware reapply: when
  the proxy owns the live config (backup/placeholder present), only the
  live backup is updated, mirroring the provider-switch path.
- The takeover backup path runs the same injection so a takeover
  release restores a config that still carries the unified routing.
- Injection refuses to activate a foreign [model_providers.custom]
  table (e.g. stale entry with a third-party base_url) to avoid routing
  ChatGPT OAuth traffic to an unknown backend.

The toggle lives under Settings → Codex App Enhancements; the
description warns that resuming old sessions across providers may fail
because encrypted_content reasoning only decrypts on the backend that
created it (upstream treats cross-provider resume as unsupported).

src/components/settings/CodexAuthSettings.tsx

@@ -1,5 +1,5 @@
 import { useTranslation } from "react-i18next";
-import { KeyRound } from "lucide-react";
+import { History, KeyRound } from "lucide-react";
 import type { SettingsFormState } from "@/hooks/useSettings";
 import { ToggleRow } from "@/components/ui/toggle-row";
 
@@ -30,6 +30,16 @@ export function CodexAuthSettings({
           onChange({ preserveCodexOfficialAuthOnSwitch: value })
         }
       />
+
+      <ToggleRow
+        icon={<History className="h-4 w-4 text-sky-500" />}
+        title={t("settings.unifyCodexSessionHistory")}
+        description={t("settings.unifyCodexSessionHistoryDescription")}
+        checked={settings.unifyCodexSessionHistory ?? false}
+        onCheckedChange={(value) =>
+          onChange({ unifyCodexSessionHistory: value })
+        }
+      />
     </section>
   );
 }