Display usage spend in CNY (¥) instead of USD ($)

[yikkuro]

Summary

The Total Spend / per-model spend figures shown in Settings → Usage were displayed with a $ prefix, implying USD. This PR converts those values to CNY and relabels the symbol to ¥.

Where the number comes from (and why the math is already correct)

MicroClaw does not compute cost itself. It calls the OpenClaw gateway RPC sessions.usage and relays totals.totalCost, byModel[].totals.totalCost, daily[].cost, and sessions[].usage.totalCost verbatim (each with || 0) in desktop/src/main.ts.

The value OpenClaw returns is USD: OpenClaw's src/utils/usage-format.ts estimateUsageCost() computes (input*price.input + output*price.output + cacheRead*… + cacheWrite*…) / 1_000_000, where prices come from the USD-denominated remote model catalog (catalog.openclaw.ai/models/v1/catalog.json). Its JSDoc says "Estimates USD usage cost" and there is a formatUsd() helper. There is no currency/FX logic anywhere in OpenClaw or MicroClaw.

Conclusion for the "verify calculation accuracy" part of #127: the arithmetic lives in OpenClaw and is correct; MicroClaw only relays it, so there is no MicroClaw-side math bug. The $ label was factually accurate, which is exactly why we can't just swap the glyph — doing so would mislabel a genuine USD value as CNY. A real USD→CNY conversion is required.

What this PR changes

1. Live exchange-rate lookup (desktop/src/exchange-rate.ts, new). Fetches the current USD→CNY rate from https://api.frankfurter.app/latest?from=USD&to=CNY (ECB data, no API key). In-memory cache with a 12h TTL. Robust fallback to a hardcoded ~7.2 constant (USD_TO_CNY_FALLBACK_RATE, clearly commented) when the fetch fails, so the UI never breaks offline. Injectable fetcher for tests, mirroring update-checker.ts.
2. Expose the rate to the renderer. New usage:get-exchange-rate IPC (+ preload usage.getExchangeRate()), and the rate is also included in the existing usage:get-stats payload (exchangeRate / currency).
3. Convert at the display boundary. SettingsView.vue (the only place spend is rendered) multiplies the relayed USD values by the rate via a single toCny() helper — no double conversion; raw USD is untouched everywhere else.
4. Relabel $ → ¥. Via a new settings.currencySymbol i18n key in both en-US.ts and zh-CN.ts, preserving the existing precision (.toFixed(4) for spend, .toFixed(2) for budget).

Validation

• desktop vitest: 653 passed (incl. 6 new exchange-rate.test.ts cases: cache hit / TTL refetch / live / fallback / stale-reuse).
• desktop/renderer vitest: 194 passed; vue-tsc typecheck clean; eslint 0 errors.
• Sanity: 0.00342 USD × 7.2 ≈ ¥0.0246.

Closes #127