feat: add default proxy selection for new accounts

[MiaoZang]

Summary

• add a global default proxy setting to system settings
• show and manage the default proxy from the proxy admin page
• preselect the configured default proxy when creating accounts
• keep the selected proxy editable before account creation

Notes

This PR intentionally does not include local custom OpenAI OAuth helper APIs such as addacc2, upacc2, or clsacc.

Tests

• go test ./internal/handler/admin -run 'Test.*Setting|Test.*DefaultProxy|Test.*Account' -count=1
• go test ./internal/service -run 'Test.*Setting|Test.*DefaultProxy|TestGatewayRequest|TestGeminiOAuth' -count=1
• pnpm exec vitest run src/components/account/__tests__/CreateAccountModal.spec.ts src/views/admin/__tests__/ProxiesView.spec.ts

[github-actions]

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[MiaoZang]

I have read the CLA Document and I hereby sign the CLA