feat: introduce cider for CHANGELOG management

[offich]

Description

Introduces cider as a dev dependency to manage CHANGELOG.md in Keep a Changelog format. The CHANGELOG is reformatted to follow the new structure, and an Unreleased section is added for upcoming changes.

What was done

• Added cider (0.2.10) as a dev dependency in pubspec.yaml
• Reformatted CHANGELOG.md to Keep a Changelog format:
  • Versioned headers changed to ## [x.x.x] - YYYY-MM-DD
  • Entries categorized under ### Added, ### Changed, ### Fixed
  • Comparison links added to the footer
• Added ## Unreleased section with the upcoming Flutter 3.38.10 update

What was NOT done

• No automation scripts for release workflows (to be handled separately)

Operation Confirmation

Setup & Execution

Run dart run cider to manage changelog entries (e.g., dart run cider bump minor).

Notes / Related URLs

• https://pub.dev/packages/cider