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/lib/api/subscription.ts

@@ -9,8 +9,16 @@ export const subscriptionApi = {
   getCodingPlanQuota: (
     baseUrl: string,
     apiKey: string,
+    // 火山方舟用账号 AK/SK 签名查询用量；其他供应商不传。
+    accessKeyId?: string,
+    secretAccessKey?: string,
   ): Promise<SubscriptionQuota> =>
-    invoke("get_coding_plan_quota", { baseUrl, apiKey }),
+    invoke("get_coding_plan_quota", {
+      baseUrl,
+      apiKey,
+      accessKeyId,
+      secretAccessKey,
+    }),
   getBalance: (
     baseUrl: string,
     apiKey: string,