feat(web-ui): config visual editor form mode

[SurviveM]

Requirement

Provide form-based editor for config.toml / settings.json / openclaw.json: dropdown for provider, slider for token limit, toggle for boolean options. Switchable "code view" to see raw text. Changes sync bidirectionally between form and code.

Verification

1. Open Claude config -> form mode -> select provider "anthropic" from dropdown -> drag slider to 200k -> switch to code view -> JSON reflects updated fields -> save -> file takes effect
2. Edit raw JSON in code view -> switch to form mode -> form fields reflect the edit
3. Toggle boolean option in form -> verify corresponding JSON value flips true/false