feat: Add search/filter input to Color Inspector sidebar (#1)

[Chizaram-Igolo]

Resolves #1

Overview

• Adds a search input at the top of the sidebar (above the file groups / list).
• Filter results in real-time.
• Search matches:
  • CSS variable label (e.g. --border)
  • Color value text (e.g. #aabbcc, rgba(0,0,0,.25))
  • File/group name
• Filtering does the following:
  • Hides non-matching rows
  • Hides file groups with zero matching rows
  • Keeps group headers visible when they still contain matches