Problem

Fixes #4500.

Status bar session cost shows ¥ at startup, then flips to $ after the first conversation turn and never recovers.

Root cause: sessionCurrency was hardcoded to "¥" in initialState, but usage events from the backend carry "$" (DeepSeek pricing config has Currency: "$"). The reducer unconditionally replaced the value:

const sessionCurrency = e.usage?.currency || s.sessionCurrency || "¥";
//                      ^^^^^^^^^^^^^^^^^^
//                      "$" from DeepSeek pricing, overwrites "¥" every turn

A naive priority swap would freeze ¥ forever, breaking USD-billing users.

Solution

Three-tier fallback, with the balance endpoint as the source of truth. Once set, sessionCurrency is never overwritten:

Balance (real billing currency — most reliable)
  ↓ fallback
Usage event (model pricing config)
  ↓ fallback
"¥" (default)

The balance endpoint already receives per-currency data from DeepSeek ("CNY", "USD"). A new Currency() method exposes the raw symbol, piped through BalanceInfo to the frontend.

Scenario coverage

Files changed

Verification

• ✅ 25 frontend test suites — zero failures
• ✅ Go internal/billing + desktop compile cleanly