Summary

Fixes #764

• Delta-based filter edge dragging: Store an anchor offset when the user grabs a filter edge, so the edge tracks mouse movement rather than snapping to the absolute cursor position. This prevents the CW low-cut from jumping to 0 Hz on the first drag event.
• Overlapping grab zone disambiguation: When both filter edges are within the 5px grab zone (common in CW's narrow filters), pick the closer edge instead of always defaulting to the low edge.
• VFO center line width: Reduce the VFO center line from 2px to 1px when a filter edge is within 4px, preventing the visual "thick line" overlap in CW mode.

Test plan

• In CW mode, grab the low-cut filter edge on the panadapter and drag — it should track smoothly without snapping to the carrier
• In CW mode at narrow zoom, verify that clicking near overlapping filter edges grabs the intended (closer) edge
• Verify the VFO center line appears thinner in CW mode where it overlaps with filter edges
• Verify SSB/AM filter edge dragging still works normally (no regression)
• Verify the VFO center line remains 2px wide in SSB/AM modes where filter edges are far from carrier

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