feat(providers): add Volcengine Ark usage query with AK/SK signing

Query coding-plan / agent-plan usage for Volcengine Ark via the
control-plane OpenAPI (open.volcengineapi.com), which requires
account-level AccessKey signing rather than the inference API key.

- Implement Volcengine Signature V4 (an AWS SigV4 variant: fixed
  canonical header order, "HMAC-SHA256" algorithm without the AWS4
  prefix, scope ending in "request", service "ark") in
  services/coding_plan.rs
- Auto-detect the plan by probing GetAFPUsage (Agent Plan, AFP
  five-hour/weekly/monthly quotas) first, falling back to
  GetCodingPlanUsage (Coding Plan, session/weekly/monthly percentages);
  a single credential covers whichever plan the account holds
- Parse the real response shape: the window label lives in the `Level`
  field; guard ResetTimestamp <= 0 (session returns -1)
- Store account-level credentials on UsageScript (accessKeyId /
  secretAccessKey), threaded through both the test command and the
  TOKEN_PLAN auto-refresh path
- Add an independent AK/SK input block with a detailed hint pointing to
  the Volcengine console -> API Access Keys
- Add the `monthly` tier label (frontend TIER_I18N_KEYS + tray
  TIER_LABEL_GROUPS + subscription service)
- Sync zh / en / ja / zh-TW locales and the usage-query docs

src/i18n/locales/en.json

@@ -1557,6 +1557,9 @@
     "accessTokenPlaceholder": "Generate in 'Security Settings'",
     "userId": "User ID",
     "userIdPlaceholder": "e.g., 114514",
+    "accessKeyId": "AccessKey ID",
+    "secretAccessKey": "SecretAccessKey",
+    "volcengineAkSkHint": "Volcengine usage query needs an account-level AccessKey ID / Secret (different from the inference API key). Create them in the Volcengine console via the top-right account menu → 'API Access Keys'.",
     "defaultPlan": "Default Plan",
     "queryFailedMessage": "Query failed",
     "queryScript": "Query script (JavaScript)",
@@ -2856,6 +2859,7 @@
     "geminiFlash": "Flash",
     "geminiFlashLite": "Flash Lite",
     "weeklyLimit": "Weekly",
+    "monthly": "Monthly",
     "copilotPremium": "Premium",
     "utilization": "{{value}}%",
     "resetsIn": "Resets in {{time}}",

src/i18n/locales/ja.json

@@ -1557,6 +1557,9 @@
     "accessTokenPlaceholder": "「Security Settings」で生成",
     "userId": "ユーザー ID",
     "userIdPlaceholder": "例: 114514",
+    "accessKeyId": "AccessKey ID",
+    "secretAccessKey": "SecretAccessKey",
+    "volcengineAkSkHint": "Volcengine の使用量照会にはアカウントレベルの AccessKey ID / Secret が必要です（推論 API キーとは別物）。Volcengine コンソール右上のアカウントメニュー →「API アクセスキー」で作成してください。",
     "defaultPlan": "デフォルトプラン",
     "queryFailedMessage": "照会に失敗しました",
     "queryScript": "照会スクリプト (JavaScript)",
@@ -2856,6 +2859,7 @@
     "geminiFlash": "Flash",
     "geminiFlashLite": "Flash Lite",
     "weeklyLimit": "週間",
+    "monthly": "月間",
     "copilotPremium": "プレミアム",
     "utilization": "{{value}}%",
     "resetsIn": "{{time}}後にリセット",

src/i18n/locales/zh-TW.json

@@ -1529,6 +1529,9 @@
     "accessTokenPlaceholder": "在「安全設定」裡產生",
     "userId": "使用者 ID",
     "userIdPlaceholder": "例如：114514",
+    "accessKeyId": "AccessKey ID",
+    "secretAccessKey": "SecretAccessKey",
+    "volcengineAkSkHint": "火山用量查詢需帳號級 AccessKey ID / Secret（與推理 API Key 不同）。請在火山引擎主控台右上角帳號選單 →「API存取金鑰」中建立。",
     "defaultPlan": "預設方案",
     "queryFailedMessage": "查詢失敗",
     "queryScript": "查詢腳本（JavaScript）",
@@ -2828,6 +2831,7 @@
     "geminiFlash": "Flash",
     "geminiFlashLite": "Flash Lite",
     "weeklyLimit": "每週",
+    "monthly": "每月",
     "copilotPremium": "進階請求",
     "utilization": "{{value}}%",
     "resetsIn": "{{time}} 後重設",

src/i18n/locales/zh.json

@@ -1557,6 +1557,9 @@
     "accessTokenPlaceholder": "在'安全设置'里生成",
     "userId": "用户 ID",
     "userIdPlaceholder": "例如：114514",
+    "accessKeyId": "AccessKey ID",
+    "secretAccessKey": "SecretAccessKey",
+    "volcengineAkSkHint": "火山用量查询需账号级 AccessKey ID / Secret（与推理 API Key 不同）。请在火山引擎控制台右上角账号菜单 →「API访问密钥」中创建。",
     "defaultPlan": "默认套餐",
     "queryFailedMessage": "查询失败",
     "queryScript": "查询脚本（JavaScript）",
@@ -2856,6 +2859,7 @@
     "geminiFlash": "Flash",
     "geminiFlashLite": "Flash Lite",
     "weeklyLimit": "每周",
+    "monthly": "每月",
     "copilotPremium": "高级请求",
     "utilization": "{{value}}%",
     "resetsIn": "{{time}}后重置",