Add Safe Max leverage control

[Sikkra]

Summary

• add a Safe Max button beside the leverage slider
• add a Safe Max HF floor setting in the Settings menu, defaulting to 1.20 and persisting in localStorage
• snap leverage down to the highest 0.1x slider step where projected HF stays at or above the configured floor

Verification

• npm run build
• git diff --check
• Local Edge/Playwright fallback check on http://127.0.0.1:5202/: default floor set Safe Max to 3.0x with HF 1.2; edited floor persisted as 1.5; Safe Max then set 2.1x with HF 1.5273; browser errors 0

Note: the Codex in-app browser stopped providing an active pane during this check, so I used the local browser fallback after attempting to reconnect it.

Closes #6