feat: DeepSeek usage panel — balance, per-request cost, daily history

[rbinar]

Focused, panel-only split of #163, per the review feedback there. This branch contains only the usage panel — nothing else.

What

• Status bar entry ($(hubot)) that opens a webview panel
• Account balance from /user/balance
• Estimated cost per request, derived from the model pricing already in consts.ts
• Persistent daily usage history (input/output tokens, request count, cost) with day-boundary reset, plus a chronological per-request breakdown and a per-model rollup
• Reset button and quick links

How usage is sourced

Usage comes solely from the provider's own request stream — the usage reported to provideLanguageModelChatResponse is fed into the tracker via a single hook in stream.ts. This is stable and authoritative; the panel reads no VS Code internal storage and persists no prompt text or chat titles.

Scope

This PR is intentionally limited to the panel. Everything else that was bundled into #163 is excluded and left to its own PRs:

• Context-window selector and model token-metadata changes → feat: add selectable context window (200K/1M) in model picker #156
• Thinking-compatibility / debug-dump changes → separate concern
• Raw-response capture callback, client error-shape changes → separate concern
• No package-lock.json churn

Diff is purely additive: 2 new files (src/balance.ts, src/tracker.ts), the panel wiring in src/runtime/lifecycle.ts, a 3-line modelId field for usage attribution in request.ts, a single recordUsage hook in stream.ts, and the views/viewsContainers contributions in package.json.

Deferred

Conversation-level grouping is omitted until VS Code exposes a stable session id to the provider API (microsoft/vscode#305853). The tracker leaves a clean seam to add it later.

Supersedes #163 (and the earlier #158).