feat: support custom provider endpoint URL

[hanneshapke]

Summary

• Adds a Custom Endpoint URL field to the OpenAI section of the Settings modal so users can point at OpenAI-compatible servers (LM Studio, Ollama, vLLM, Together, etc.). Validated to require an http(s):// scheme and persisted in the existing Electron config alongside model and the encrypted API key.
• Wires the persisted provider config (API keys + base URLs for all providers) into the Go backend by injecting them as the env vars loadApplicationConfig already understands (OPENAI_API_KEY, OPENAI_BASE_URL, …) when the Go binary is spawned.
• Adds a restart-backend IPC + a stopGoBinaryAsync / restartGoBinary pair, and the modal calls it after a successful save so changes apply without restarting the whole app.

Test plan

• Open Settings → OpenAI: enter https://api.openai.com/v1 (or an LM Studio/Ollama URL) and a key, save, verify the success message and that requests now hit the configured endpoint.
• Save with an invalid URL (no scheme) → expect a validation error from the IPC handler.
• Save with EXTERNAL_BACKEND=true → expect the "externally managed" error from restart-backend.
• Confirm previously-saved API keys still work after the restart path is exercised.

User Interface

[github-actions]

Warning

This PR touches 3+ distinct areas of the codebase.

Consider splitting into smaller, focused PRs — each covering a single semantic type.
This makes reviews easier and keeps the git history clean.

Categories found:

code:

• src/backend/config/config.go
• src/backend/main.go
• src/backend/providers/custom.go
• src/backend/providers/provider.go
• src/backend/proxy/handler.go
• src/backend/server/server.go
• src/frontend/src/components/modals/SettingsModal.tsx
• src/frontend/src/components/privacy-proxy-ui.tsx
• src/frontend/src/electron/electron-main.js
• src/frontend/src/electron/electron-preload.js
• src/frontend/src/electron/electron.d.ts
• src/frontend/src/hooks/useElectronSettings.ts
• src/frontend/src/types/provider.ts
• src/frontend/src/utils/providerHelpers.ts

chore:

• package-lock.json

test:

• src/backend/config/config_test.go
• src/backend/providers/provider_test.go
• src/backend/providers/providers_test.go
• src/backend/proxy/handler_test.go

[nmartorell]

@hanneshapke code changes looks reasonable :)
Going to perform a quick functional test now before approving.

[nmartorell]

Functional tests for new custom provider all worked as expected.