Add Overview UI icon set, Icon Button prefab and hover/disabled ring

- 7 generated icons (settings, statistics, paint, freecam, thirdperson,
  firstperson v3 viewfinder, spectator eye) + hand-painted hover ring,
  full-res PNGs imported as sprites (max size 256)
- Icon Background texture (user asset)
- IconButtonHover: shows the Hover child on pointer hover or while the
  button is non-interactable (doubles as active-mode marker)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/Scripts/Components/UI/IconButtonHover.cs.meta

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