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/i18n/locales/en.json

@@ -670,6 +670,8 @@
     "codexAuth": "Codex App Enhancements",
     "preserveCodexOfficialAuthOnSwitch": "Keep official login when switching third-party providers",
     "preserveCodexOfficialAuthOnSwitchDescription": "When enabled, you can use the Codex app's official plugins, mobile remote control, and other features while using third-party APIs.",
+    "unifyCodexSessionHistory": "Unified Codex session history",
+    "unifyCodexSessionHistoryDescription": "When enabled, the official subscription runs under the shared \"custom\" provider id, so future official sessions appear in the same history list as third-party sessions (existing sessions are not migrated). Note: resuming an old session across providers may fail because its encrypted_content reasoning can only be decrypted by the backend that created it.",
     "appVisibility": {
       "title": "Homepage Display",
       "description": "Choose which apps to show on the homepage",

src/i18n/locales/ja.json

@@ -670,6 +670,8 @@
     "codexAuth": "Codex アプリ拡張",
     "preserveCodexOfficialAuthOnSwitch": "サードパーティ切替時に公式ログインを保持",
     "preserveCodexOfficialAuthOnSwitchDescription": "オンにすると、サードパーティ API を使用する際に Codex アプリの公式プラグインやスマホからのリモート操作などの機能を利用できます。",
+    "unifyCodexSessionHistory": "Codex セッション履歴を統一",
+    "unifyCodexSessionHistoryDescription": "オンにすると、公式サブスクリプションも共有の custom プロバイダー ID で動作し、今後の公式セッションはサードパーティのセッションと同じ履歴リストに表示されます（既存セッションは移行しません）。注意：プロバイダーをまたいで古いセッションを再開すると、encrypted_content の推論内容を相手のバックエンドが復号できず、再開に失敗する場合があります。",
     "appVisibility": {
       "title": "ホームページ表示",
       "description": "ホームページに表示するアプリを選択",

src/i18n/locales/zh-TW.json

@@ -670,6 +670,8 @@
     "codexAuth": "Codex 應用增強",
     "preserveCodexOfficialAuthOnSwitch": "切換第三方時保留官方登入",
     "preserveCodexOfficialAuthOnSwitchDescription": "開啟後可以在使用第三方 API 的時候使用 Codex 應用的官方外掛、手機遠端操作等功能",
+    "unifyCodexSessionHistory": "統一 Codex 會話歷史",
+    "unifyCodexSessionHistoryDescription": "開啟後，官方訂閱將以共享的 custom 供應商標識執行，未來的官方會話與第三方會話出現在同一歷史清單中（不遷移既有會話）。注意：跨供應商繼續舊會話時，對方後端可能無法解密會話中的 encrypted_content 推理內容，導致繼續失敗",
     "appVisibility": {
       "title": "主頁面顯示",
       "description": "選擇在主頁面顯示的應用程式",

src/i18n/locales/zh.json

@@ -670,6 +670,8 @@
     "codexAuth": "Codex 应用增强",
     "preserveCodexOfficialAuthOnSwitch": "切换第三方时保留官方登录",
     "preserveCodexOfficialAuthOnSwitchDescription": "开启后可以在使用第三方 API 的时候使用 Codex 应用的官方插件、手机远程操作等功能",
+    "unifyCodexSessionHistory": "统一 Codex 会话历史",
+    "unifyCodexSessionHistoryDescription": "开启后，官方订阅将以共享的 custom 供应商标识运行，未来的官方会话与第三方会话出现在同一历史列表中（不迁移既有会话）。注意：跨供应商继续旧会话时，对方后端可能无法解密会话中的 encrypted_content 推理内容，导致继续失败",
     "appVisibility": {
       "title": "主页面显示",
       "description": "选择在主页面显示的应用",