Fix Ollama endpoint validation and model loading

[Finesssee]

Summary

• move Ollama endpoint probing behind one shared path instead of mixing browser fetches and provider-specific behavior
• add a Rust-side /api/tags probe command so custom Ollama URLs work without depending on the frontend HTTP allowlist
• reuse that probe in settings, the model selector, and dynamic model loading so the UI stops rejecting valid endpoints

Testing

• bun fix
• bun test
• bun typecheck
• cargo test --manifest-path src-tauri/Cargo.toml ollama -- --nocapture

Fixes #372