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/services/subscription.rs

@@ -307,6 +307,11 @@ pub const TIER_SEVEN_DAY_SONNET: &str = "seven_day_sonnet";
 /// 写入、tray 渲染、commands::provider 扁平化三处共用同一标识。
 pub const TIER_WEEKLY_LIMIT: &str = "weekly_limit";
 
+/// 月窗口 tier 名。火山方舟 Agent Plan / Coding Plan 有 5h / 周 / 月 三个展示
+/// 窗口（Kimi / MiniMax 只有 5h + 周），月窗口共用此标识；前端 `TIER_I18N_KEYS`
+/// 映射到 `subscription.monthly`。
+pub const TIER_MONTHLY: &str = "monthly";
+
 /// Gemini 用量分组名称（按模型而非时间窗口）。`classify_gemini_model` 输出。
 pub const TIER_GEMINI_PRO: &str = "gemini_pro";
 pub const TIER_GEMINI_FLASH: &str = "gemini_flash";