Fix: eyedropper and flood selection to reapply when threshold changes

[reagan99]

Summary
This PR fixes an issue where the Eyedropper and Flood selection tools did not update the current selection when the threshold value was changed after clicking. Previously, users had to click again for the new threshold to take effect. With this change, the selection is immediately recomputed whenever the threshold changes.

Changes

• Eyedropper: store the last click state and re-run color matching on threshold updates
• Flood: store the last operation (set/add/remove) and re-run flood selection on threshold updates
• Bind threshold UI to input events for real-time updates and clamp values to [0, 1]

Files Changed

• src/tools/eyedropper-selection.ts
• src/tools/flood-selection.ts

Test Plan

1. Use Eyedropper selection, then change threshold without clicking again → selection updates immediately
2. Use Flood selection, then change threshold without clicking again → selection updates immediately