fix: Reduce drag scroll speed to prevent overshooting columns (#1110)

[BradGroux]

Problem

When dragging cards across a Kanban board with many columns, the horizontal scroll speed is far too fast, making it nearly impossible to drop cards into adjacent columns. The board scrolls all the way to the end instead of allowing precise placement.

This affects both desktop and mobile users, as reported in #1110 and confirmed by multiple users in #1166.

Root Cause

The scrollStrengthModifier constant in ScrollManager.ts was set to 8. At 60fps, this produces scroll speeds up to 480px/sec when the cursor is near the edge of the scroll zone — far too fast for accurate card placement.

Fix

Reduced scrollStrengthModifier from 8 to 2.5.

	Old (8)	New (2.5)
Max speed (at edge)	~480 px/sec	~150 px/sec
Mid-zone speed	~240 px/sec	~75 px/sec
Min speed (deep zone)	~0 px/sec	~0 px/sec

The new value of 2.5 provides a good balance:

• Fast enough to traverse large boards without excessive dragging
• Slow enough to accurately drop cards one or two columns over
• Consistent with the user testing in [Bug]: Scroll speed too fast #1110 where scrollStrengthModifier=1 was confirmed as usable (we use a slightly higher value for better large-board ergonomics)

Testing

• Tested with 7+ column boards
• Drag from first column to adjacent columns is now controllable
• Drag across the entire board still completes in a reasonable time

Fixes #1110