feat: add OAuth proxy controls to Web UI

[StarsWhere]

Summary

Adds OAuth proxy controls to the Web UI so users can provide a one-time proxy for OAuth login and optionally fill that field from the current global proxy.

Closes #245

Key changes

• Adds a shared OAuth proxy URL input at the top of the OAuth page.
• Sends explicit OAuth proxy values as proxy_url when starting OAuth login.
• Adds a global proxy fill action that copies the configured global proxy into the one-time OAuth proxy field.
• Keeps the OAuth proxy input in memory only and avoids localStorage persistence.
• Adds localized text for zh-CN, zh-TW, en, and ru.

Testing

• Locale JSON parse check
• npm run lint
• npm run build

[gemini-code-assist]

Code Review

This pull request introduces the ability to configure a proxy for OAuth logins, allowing users to specify a proxy URL or use the global proxy settings for server-side operations. Changes include new UI components on the OAuth page, updated localization files, and modifications to the OAuth API service to handle the proxy parameter. A suggestion was made to simplify the logic for passing authentication options by using shorthand property names and removing redundant undefined checks.

[gemini-code-assist]

The logic for constructing the options object can be simplified using shorthand property names. Additionally, projectId || undefined is redundant because projectId is already defined as string | undefined and is guaranteed not to be an empty string at this point based on the logic at lines 242-246.

Suggested change

	provider === 'gemini-cli'
	? { projectId: projectId || undefined, proxyUrl }
	: proxyUrl
	? { proxyUrl }
	: undefined
	provider === 'gemini-cli'
	? { projectId, proxyUrl }
	: proxyUrl
	? { proxyUrl }
	: undefined

[StarsWhere]

Follow-up pushed in aa242e5 addressing the review suggestion by simplifying the Gemini OAuth options construction.\n\nTesting:\n- npm run lint\n- npm run build