feat(providers): implement real API key validation with OpenRouter support

- Replace mock API key validation with actual API calls to verify keys
- Add validateApiKeyWithProvider() with provider-specific implementations
- Support Anthropic, OpenAI, Google, and OpenRouter validation
- Add OpenRouter as a new provider option in setup wizard and settings
- Fix setup page to call real validation instead of mock length check
- Allow validation during setup before provider is saved
- Return user-friendly error messages instead of raw API errors

src/stores/providers.ts

@@ -10,7 +10,7 @@ import { create } from 'zustand';
 export interface ProviderConfig {
   id: string;
   name: string;
-  type: 'anthropic' | 'openai' | 'google' | 'ollama' | 'custom';
+  type: 'anthropic' | 'openai' | 'google' | 'openrouter' | 'ollama' | 'custom';
   baseUrl?: string;
   model?: string;
   enabled: boolean;