feat: add drag-and-drop reordering for gem slots and gem selector improvements

[jay9297]

Summary

• Add drag-and-drop reordering for gem slots in the Skills tab
• Add gem selector improvements: support gem filter for main slot, active-only gem selection
• Disable level and quality controls for support gems (PoE2 support gems have tiers, not levels/quality)

Details

Drag & Drop

Implements dead-reckoning drag reordering with 5px threshold for insertion detection. Renders an insertion line indicator between slots during drag. Properly resyncs all slot widgets after reorder via gemList reorganization.

Gem Selector

• activeOnly filter added to GemSelectClass constructor for slot 1, preventing support gems from appearing as main active skills
• "x" button on main skill slot opens gem selector instead of removing the required active gem

Support Gem Level/Quality Disable

Verified via poe2wiki.net: PoE2 support gems use tiers (I/II/III), not levels or quality. All 557 support gems have naturalMaxLevel = 1 and empty qualityStats blocks. The level/quality controls are now properly disabled when a support gem is selected.

Test plan

• Drag a gem slot to a new position and verify correct reordering
• Verify insertion line indicator appears during drag
• Verify support gems do NOT appear in the main active skill slot dropdown
• Verify "x" on main slot opens gem selector (does not remove gem)
• Verify level and quality controls are disabled for support gems, enabled for skill gems