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-tauri/src/commands/coding_plan.rs

@@ -4,6 +4,15 @@ use crate::services::subscription::SubscriptionQuota;
 pub async fn get_coding_plan_quota(
     base_url: String,
     api_key: String,
+    // 火山方舟用控制面 AK/SK 签名查询用量；其他供应商不传，沿用 api_key。
+    access_key_id: Option<String>,
+    secret_access_key: Option<String>,
 ) -> Result<SubscriptionQuota, String> {
-    crate::services::coding_plan::get_coding_plan_quota(&base_url, &api_key).await
+    crate::services::coding_plan::get_coding_plan_quota(
+        &base_url,
+        &api_key,
+        access_key_id.as_deref(),
+        secret_access_key.as_deref(),
+    )
+    .await
 }