Implement multi-point fan curve editor (US-B2)

[naufaldi]

Problem

PRD US-B2: No curve editor, no multi-point interpolation, no named curve presets. Currently only 2-point linear interpolation exists.

Acceptance Criteria

• Graph editor UI for custom curves (temperature → fan speed)
• Support at least 4 curve points (idle, low, medium, high)
• Preset curves: Silent, Balanced, Performance, Maximum
• Apply different curves to different fans
• Save custom curves with names

Notes

Large feature. The existing 2-point sensor-based control works for basic use cases.