feat: support custom Claude model

[metowolf]

Summary

• Add support for specifying custom Claude models when spawning sessions
• Infer custom Claude model from the model value automatically
• Update web UI to allow custom model input with proper i18n labels
• Fix related model options test

Changes

• Spawn: Support custom model type for Claude agent, allowing users to input any model name (e.g., custom model versions or preview builds)
• Model inference: Automatically detect and handle custom Claude model strings that don't match presets
• Web UI: Add custom model input field with translated placeholder and validation messages
• Tests: Update model options test to include the new custom option

Test plan

• Model options test passes with custom option included
• Manual testing of custom model input in web UI
• Verify custom model is correctly passed to Claude agent on spawn

[github-actions]

Findings

• No issues found.

Summary

• Review mode: initial
• Reviewed the full diff for runner-spawned Claude model propagation and the NewSession custom model UI. Residual risk: no automated web interaction test covers the custom-model field/validation path.

Testing

• Not run (automation)

HAPI Bot