Add cider and reformat CHANGELOG to Keep a Changelog style

[offich]

Description

Add cider as a dev dependency for changelog management and reformat CHANGELOG.md to follow the Keep a Changelog format with versioned headers, dated entries, categorized sections, and link references.

What was done

• Added cider: 0.2.10 to dev_dependencies in pubspec.yaml
• Reformatted CHANGELOG.md to Keep a Changelog style:
  • Version headers changed to ## [x.y.z] - YYYY-MM-DD format
  • Added ### Added / ### Fixed / ### Changed sections per version
  • Added ## Unreleased section at the top
  • Added version comparison link references at the bottom

What was NOT done

• Did not change the content of changelog entries, only formatting

Operation Confirmation

Prerequisites & Steps

1. Run flutter pub get to install cider
2. Use dart run cider to manage changelog entries going forward

Notes & Related URLs

N/A