feat(client): C 端 1-2 账号链路（邮箱登录/注册 + 令牌与 401 + 匿名/登录分叉）

为什么：客户端此前完全没有账号概念（mine 页的"登录"只是本地 demo 开关），
而受限动作（收藏/评论）要对到具体账号。本批把账号链路接上，为后续互动、进度同步、下载打底。

- AuthApi + AuthController + AuthSessionStore：注册/登录/登出/刷新，令牌与设备号存 shared_preferences
  （用 SharedPreferencesAsync，与 engagement 仓库同一套 API）
- ApiClient：统一带 Authorization 与 X-Device-Id；401 自动刷新一次并重试，刷新失败清登录态走 onUnauthorized
- 登录/注册页（pages/auth，沿用 binding/logic/view 与现有视觉）：邮箱格式、密码 8-64 位含字母数字本地先校验，
  错误提示走 l10n；登录/注册一键切换
- 匿名/登录分叉：首页、发现、详情保持匿名可看；收藏/点赞等受限动作走 ensureSignedIn() 弹底部提示引导登录
- mine 页展示真实账号（昵称 + uid），拿不到才回落到演示文案
- 测试：新增 auth 控制器（注册落盘/401 刷新重试/刷新失败清会话/登出）与登录页（校验/切换）用例；
  widget_test 改为预置本地会话（账号不再是本地开关）；修掉 playback 用例对 host 大小写的环境依赖断言

门禁：flutter analyze 0 问题；flutter test 148 全过。

lib/l10n/app_en.arb

@@ -96,6 +96,8 @@
   "lockedBody": "Unlocking will be available after store and verification setup. This demo does not process payments.",
   "playbackError": "This clip could not be played.",
   "catalogError": "The catalog could not be loaded.",
+  "detailError": "The episode list could not be loaded.",
+  "noMediaNotice": "This episode has no playable media yet.",
   "noStories": "No stories available yet.",
   "retry": "Retry",
   "backToSeries": "Back to series",
@@ -130,5 +132,53 @@
   "continueWatchingHeading": "CONTINUE WATCHING",
   "preferencesHeading": "PREFERENCES",
   "brandMark": "NONTONKI · DEMO",
-  "shareFooter": "Nontonki demo"
+  "shareFooter": "Nontonki demo",
+  "shortsTab": "Shorts",
+  "myListTab": "My List",
+  "watchFull": "Watch Full",
+  "swipeForNext": "Swipe up for next",
+  "clearScreen": "Clear screen",
+  "exitClearScreen": "Exit clear screen",
+  "logIn": "Log in",
+  "logInHint": "Keep your coins and list",
+  "rewards": "Rewards",
+  "forYou": "For You",
+  "hot": "Hot",
+  "newest": "New",
+  "favorite": "Favorite",
+  "myAccount": "My account",
+  "coins": "Coins",
+  "topUpWallet": "Top Up Wallet",
+  "vipMembership": "VIP Membership",
+  "notActive": "Not Active",
+  "joinVip": "Join VIP",
+  "earnRewards": "Earn Rewards",
+  "freeBonus": "Get Free Bonus",
+  "watchHistory": "Watch History",
+  "settings": "Settings",
+  "signOut": "Sign Out",
+  "mockUserName": "Vicky sheng",
+  "mockUserId": "ID: M1CZWVYE",
+  "mockNotice": "Sample data only.",
+  "topBadge": "TOP 1",
+  "signIn": "Sign In",
+  "signUp": "Sign Up",
+  "authSignInTitle": "Welcome back",
+  "authSignUpTitle": "Create your account",
+  "authSubtitle": "Sign in with your email and password.",
+  "authEmailLabel": "Email",
+  "authPasswordLabel": "Password",
+  "authPasswordHint": "At least 8 characters, letters and numbers",
+  "authEmailRequired": "Enter your email",
+  "authEmailInvalid": "That email doesn't look right",
+  "authPasswordRequired": "Enter your password",
+  "authPasswordLength": "Password must be 8-64 characters",
+  "authPasswordWeak": "Password needs both letters and numbers",
+  "authNetworkError": "Network error, please try again",
+  "authSwitchToSignUp": "New here? Create an account",
+  "authSwitchToSignIn": "Already have an account? Sign in",
+  "authGateTitle": "Sign in to continue",
+  "authGateMessage": "Favorites and comments need an account.",
+  "authGateAction": "Sign In",
+  "cancel": "Not now"
 }