refactor(desktop): extract Usage settings into a renderer feature slice

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Problem

Settings → Usage (apps/desktop/src/renderer/settings/usage-settings-page.tsx and its tabs) is still legacy-zone renderer code inside the AppShell closure. Since the renderer-architecture ratchet landed (#4088, the R1 foundation of #3439), the legacy AppShell closure is frozen: no new file may enter it, and no legacy file's dependency count may grow (only shrink).

This blocks net-new Usage functionality. Concretely, #2015 / PR #4164 (editable Runtime Host-backed pricing overrides) must add a Pricing tab inside the Usage settings page — #2015 explicitly requires it stay a Usage tab, not a new top-level Settings destination. Adding that tab forces usage-settings-page.tsx to import the pricing surface: a +1 dependency on a frozen file with no existing code to remove as an offset (unlike the Workbar/Goals extractions, whose feature import replaced removed code, netting ≤0). The ratchet correctly rejects it, and there is currently no features/usage slice to attach to.

Usage/Settings was never a tracked slice under #3439 (which covered Workbar, Goals, Module Hub, Session Navigation, Task Entry, Conversation, …). This issue tracks that missing slice.

Proposed direction

Extract the Usage settings surface into a renderer feature src/renderer/features/usage/, following the reference boundary from #3439 (model / controller / ui / ports / desktop adapter / testing):

• The four existing tabs (Activity / Providers / Models / Tools), the range control, and the summary metrics move behind the feature's ports and controller.
• The Pricing override editor (feat(desktop): add editable Runtime Host-backed pricing settings #2015) lives inside the feature rather than as legacy debt.
• usage-settings-page.tsx (legacy) delegates to the feature's public API via a single migration-dependency edge, keeping closure debt flat-or-lower.
• Services are provided from composition/desktop-feature-services.tsx; the Desktop adapter lives under platform/desktop/.

Scope

• features/usage/: model/controller/ui/ports for the Usage stats surface + the Pricing override editor.
• Desktop adapter + composition provider wiring.
• Retire the legacy usage-settings-page.tsx internals.

Invariants (per #3439)

No product, visual, IPC, storage-schema, copy, or shortcut changes. The Usage stats behavior, range persistence, and the pricing load/mutate/CAS semantics from #2015 must remain compatible.

Refs

• Foundation / ratchet: refactor(desktop): make AppShell a renderer composition root #3439, refactor(desktop): lock renderer root architecture #4088
• Blocked feature: feat(desktop): add editable Runtime Host-backed pricing settings #2015 (PR feat(desktop): add pricing override management UI #4164)

Non-goals

• No product/visual/IPC/copy changes.
• No pricing product-direction change (that is feat: user-overridable model facts (context window etc.) — models.json-style override layer #2330 / the feat(desktop): add editable Runtime Host-backed pricing settings #2015 pause) — this issue is purely the renderer-ownership migration that unblocks it.

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_{中文摘要：设置 → 使用统计 仍是 legacy 区、被架构棘轮（#4088）冻结，无法新增依赖；而 #2015/#4164 的定价必须做成 Usage 标签页（净新增，无可抵消），于是撞墙。本 issue 跟踪按 #3439 的 model/controller/ui/ports/adapter 模式，把整个 Usage 设置面（4 个标签页 + 数据加载 + 定价 tab）抽取为 features/usage 切片，从而解除阻塞。行为/文案/IPC 不变。}

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