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/components/SubscriptionQuotaFooter.tsx

@@ -33,6 +33,8 @@ export const TIER_I18N_KEYS: Record<string, string> = {
   gemini_flash_lite: "subscription.geminiFlashLite",
   // Token Plan（five_hour 已在上方官方映射中）
   weekly_limit: "subscription.sevenDay",
+  // 火山方舟 Agent Plan / Coding Plan 的月窗口
+  monthly: "subscription.monthly",
   // GitHub Copilot
   premium: "subscription.copilotPremium",
 };