Add Beginner Mode leverage cap

[Sikkra]

Summary

• add persisted Beginner Mode controls to desktop settings and mobile sidebar
• cap leverage controls at 2x in Beginner Mode and pass the mode into Blend builders for an extra clamp before request construction
• relabel HF surfaces as Safety Score while enabled
• hide raw c_factor, pool/strategy contract links, broker settings, and high-risk slider zones from Beginner Mode

Closes #7.

Verification

• npm run build in frontend/ (passes; existing Vite chunk-size warning)
• Playwright/Edge smoke test on local Vite server: toggle persists through reload, slider/input max <= 2, Safety Score label appears, c_factor/pool contract/high-risk zones are hidden
• git diff --check