fix(desktop): wallet balance follows actual recharge currency

[clearnature]

Summary

Wallet balance display followed the UI language guess (zh→CNY, en→USD),
which is wrong for overseas Chinese users who recharge USD but use a zh
interface. Also, picking CNY/USD in Settings did not refresh the
status-bar balance readout.

Changes

1. Balance.PrimaryCurrency() (internal/billing/balance.go)
   Returns the largest reported balance's currency. DeepSeek returns one
   Info per currency; the largest balance is the one the user actually
   recharged, so display should follow it rather than a language guess.

2. BalanceForTab (desktop/app.go)
   When desktop currency is auto (no explicit choice), display follows
   PrimaryCurrency() instead of the UI-language guess. An explicit
   CNY/USD choice in Settings still wins.

3. applyDeepSeekOfficialDefaultPricingWithOverride
   (internal/config/pricing.go)
   Match provider-wide price against p.ModelList() so
   deepseek-responses (models list + provider-wide price fallback, empty
   p.Model) also gets converted to the selected CNY/USD table. Before
   this, a USD default stayed in dollars even with [desktop] currency = CNY.

4. useController.ts (desktop frontend)
   Refresh the wallet readout on reasonix:model-catalog-changed, which
   SettingsPanel dispatches after applying currency/model changes. Before
   this, picking CNY/USD in Settings did not update the status bar.

Verification

• go test ./internal/billing/ ./internal/config/ - all pass
• New: TestPrimaryCurrencyFollowsLargestBalance,
  TestApplyDeepSeekOfficialPricingOverridesProviderWidePrice
• Frontend tsc --noEmit - clean

Cache-impact: low - config/billing/desktop display only; no provider wire
or system-prompt changes.
Cache-guard: go test ./internal/billing/ ./internal/config/ -count=1

Documentation-impact: none - internal display logic, no user-facing docs.

System-prompt-review: none - no system-prompt or boot surface changes.

[Copilot]

Pull request overview

This PR fixes desktop wallet balance and pricing-currency behavior so the displayed wallet currency follows the user’s actual recharge currency (when desktop currency is set to auto), and so changing CNY/USD in Settings refreshes the status-bar wallet readout and correctly converts DeepSeek official default pricing even when a provider uses a model list with a provider-wide price fallback.

Changes:

• Add Balance.PrimaryCurrency() to select the currency associated with the largest reported balance entry.
• Update desktop balance display logic to prefer the actual recharge currency when desktop currency is auto, and refresh the balance readout when Settings applies model/currency changes.
• Fix DeepSeek official pricing override logic to also apply to provider-wide price fallbacks when using a models list; add regression tests.

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
internal/config/pricing.go	Adjust DeepSeek official pricing override to handle provider-wide price when models list is used.
internal/config/backfill_test.go	Add regression test ensuring provider-wide DeepSeek pricing gets overridden to selected currency table.
internal/billing/balance.go	Add PrimaryCurrency() to infer display currency from the largest reported balance.
internal/billing/balance_test.go	Add test coverage for PrimaryCurrency() selection behavior.
desktop/app.go	Prefer actual recharge currency for balance display when desktop currency is auto.
desktop/frontend/src/lib/useController.ts	Refresh wallet balance readout when Settings dispatches the model-catalog-changed event.

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[Copilot]

Pull request overview

Copilot reviewed 6 out of 6 changed files in this pull request and generated no new comments.

Suppressed comments (1)

desktop/app.go:6758

• err is reused for the config-load call after it already held the balance-fetch error earlier in this function. Reassigning the same err name for an unrelated operation is easy to misread and makes future edits riskier (e.g., accidentally checking the wrong error). Use a distinct name like cfgErr and pass that through.

	cfg, _, err := a.loadDesktopUserConfigForView()
	currency := a.balanceDisplayCurrency(cfg, err)
	if actual := b.PrimaryCurrency(); actual != "" {
		if err == nil && cfg != nil && cfg.DesktopCurrency() == "" {
			currency = actual