feat: add subscription & usage page (pi-sub-core based)

[Dwsy]

Background

We need a dedicated page in Pi Session Manager to display subscription and usage information.

Reference package: https://www.npmjs.com/package/@marckrenn/pi-sub-core

Why

• Avoid reinventing provider usage aggregation.
• Reuse the existing pi-sub-core data model / cache output.
• Give users a quick visual view of subscription status + usage windows.

Scope

• Add a new UI page for subscription/usage summary.
• Add backend command(s) to read normalized usage snapshots from local sub-core cache.
• Handle missing cache gracefully with actionable guidance.

Acceptance Criteria

• New page is reachable from app navigation.
• Page shows provider, updated time, and usage windows (used/limit/percent).
• No crash when cache file is missing; shows friendly empty state.
• Works in both desktop and headless API mode via command dispatch.

[Dwsy]

已提交实现 PR：#5\n- 新增 Subscription & Usage 页面\n- 后端新增 get_subscription_usage（读取 sub-core cache）\n- 缺失缓存时给出引导提示\n\n请在 PR #5 review。