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/provider.rs

@@ -251,6 +251,14 @@ pub struct UsageScript {
     #[serde(skip_serializing_if = "Option::is_none")]
     #[serde(rename = "codingPlanProvider")]
     pub coding_plan_provider: Option<String>,
+    /// 火山方舟控制面 OpenAPI 的 AccessKey ID（用量查询签名用，与推理 Key 是两套凭据）
+    #[serde(skip_serializing_if = "Option::is_none")]
+    #[serde(rename = "accessKeyId")]
+    pub access_key_id: Option<String>,
+    /// 火山方舟控制面 OpenAPI 的 SecretAccessKey（同上）
+    #[serde(skip_serializing_if = "Option::is_none")]
+    #[serde(rename = "secretAccessKey")]
+    pub secret_access_key: Option<String>,
 }
 
 /// 用量数据