feat: Safe Max button with configurable HF floor (#6)

[Escelit]

Summary

One-click Safe Max button that snaps the leverage slider to the highest
value where projected HF ≥ a user-configured floor.

Changes

• Safe Max button added inline next to the leverage slider; walks down
  from slider max in 0.1× steps and picks the highest leverage where
  hfForLeverage >= floor
• HF floor input added to settings dropdown (default 1.2, min 1.01)
• Floor persisted in localStorage under key safeMaxHfFloor

Acceptance criteria

• Clicking Safe Max snaps slider to max leverage where projected HF >= floor
• Floor is editable in settings menu and persists across reloads

Closes #6

[drips-wave]

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