fix(desktop): send provider currency in Meta to prevent session-start flip (#4546)

[ashishexee]

Problem

#4546 reports that the status bar cost display shows inconsistent currencies:

• Before session starts (home page): ¥0.0000 (CNY)
• During session (after first usage event): $0.0000 (USD)

The root cause is two hardcoded defaults that disagree:

• useController.ts:119 — sessionCurrency: "¥" (hardcoded CNY in frontend initial state)
• config.go:1271 — DeepSeek pricing: Currency: "$" (USD)

The sessionCurrency field only updates when a usage event arrives from the backend (line 510). Before that, it stays at the hardcoded "¥" default.

Fix

Thread the provider's currency symbol from config through Controller to Meta, so the frontend knows the currency at session start — before any usage event.

Backend (Go):

• internal/control/controller.go — Add currencySymbol field to Controller with CurrencySymbol() getter; add CurrencySymbol to Options struct
• internal/boot/boot.go — Set CurrencySymbol: entry.Price.Symbol() in controller options (nil-safe: Pricing.Symbol() returns "¥" for nil/empty)
• desktop/app.go — Add Currency string to Meta struct; populate in MetaForTab() with nil guard on tab.Ctrl

Frontend (TypeScript):

• types.ts — Add currency?: string to Meta interface
• useController.ts — case "meta" sets sessionCurrency from meta.currency; case "reset" preserves sessionCurrency; sameMeta() includes currency in comparison

Verification

go vet ./internal/control/... ./internal/boot/...
(cd desktop && go vet ./...)
go test ./internal/control/... -count=1 -timeout 120s
go test ./internal/boot/... -count=1 -timeout 120s
(cd desktop && go test ./... -run "TestMeta|TestTab|TestWire" -count=1)
gofmt -l internal/control/controller.go internal/boot/boot.go desktop/app.go

Fixes #4546