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/config/codingPlanProviders.ts

@@ -11,7 +11,7 @@ import { TEMPLATE_TYPES } from "@/config/constants";
 
 export interface CodingPlanProviderEntry {
   /** 与后端 QuotaTier 的 `codingPlanProvider` 取值对齐 */
-  id: "kimi" | "zhipu" | "minimax" | "zenmux";
+  id: "kimi" | "zhipu" | "minimax" | "zenmux" | "volcengine";
   /** UsageScriptModal 下拉显示用 */
   label: string;
   /** base_url 匹配规则 */
@@ -35,6 +35,14 @@ export const CODING_PLAN_PROVIDERS: readonly CodingPlanProviderEntry[] = [
     label: "ZenMux",
     pattern: /zenmux\./i,
   },
+  {
+    // 火山方舟 Agent Plan / Coding Plan。base_url 形如
+    // ark.cn-beijing.volces.com/api/coding[/v3]；与后端 detect_provider 的
+    // `volces.com/api/coding` 子串判断同效。
+    id: "volcengine",
+    label: "火山方舟 (Volcengine)",
+    pattern: /volces\.com\/api\/coding/i,
+  },
 ] as const;
 
 /** 根据 Base URL 自动检测 Coding Plan 供应商；未命中返回 null */