feat: add a "Custom (OpenAI Compatible)" AI provider option

[wynn5a]

Summary

Add a "Custom (OpenAI Compatible)" AI provider option that allows connecting to any OpenAI-compatible API endpoint (Azure OpenAI, Groq, Together AI, etc.). Also improves error reporting for AI connection tests across all providers — test failures now surface the actual error message instead of a generic "Connection failed".

Changes

• New customProvider.ts — OpenAI-compatible provider supporting configurable base URL, API key, and model name
• Extended AiProvider union type and providerManager to include "custom" provider with full lifecycle (create, cache, clear)
• Added AiTestResult type ({ ok: boolean; error?: string }) replacing the previous boolean return from testConnection() across all 6 providers
• Settings UI: new "Custom Provider" section with Base URL, API Key, and Model Name fields, plus Save and Test Connection buttons
• Settings UI: "Test Connection" now auto-saves form values before testing (Ollama + cloud providers) and displays truncated error details on failure
• Updated all existing provider tests (copilotProvider.test.ts, ollamaProvider.test.ts, aiService.test.ts, writingStyleService.test.ts) and shared mocks to match the new AiTestResult return shape

Type of Change

• New feature
• Bug fix
• Enhancement (improving existing feature)
• Refactor (no behavior change)
• Documentation
• CI/Build

Testing

• Existing tests pass (npm run test)
• New tests added (if applicable)
• Manually tested

Screenshots

[bilalbayasut]

please would you mind review this @avihaymenahem , can't wait :)