feat: ease context popup selection movement

[Azganoth]

Summary

The context popup followed a changing selection in whole steps, which made repeated Shift+Arrow movement read as a series of placements rather than one object following the text.

• Ease the Radix wrapper between positions produced by subsequent selection requests.
• Cancel selection easing as soon as scrolling begins so the popup continues tracking its text immediately.
• Keep the transition inactive during initial placement and while the popup holds focus, preserving opening and pinning behavior.
• Disable the new movement when prefers-reduced-motion is active.
• Record the user-facing change in the changelog.

Related Issue

Closes #172

Verification

Verified in EditorContextPopup.test.tsx: initial placement does not enable selection easing under React Strict Mode, later selection requests do, scrolling cancels it immediately, and a popup holding focus remains pinned.

Manually verified in the desktop app on Windows:

1. Selection movement eases under repeated and held Shift+Arrow.
2. Scrolling follows the anchored text immediately without lag.
3. A newly opened popup does not animate from its parked position.
4. A popup holding focus remains pinned.
5. Disabling Windows animation effects makes repositioning immediate.

Notes

Reduced-motion handling applies only to the new repositioning transition. Existing popup open and close animations remain unchanged and out of scope.