chore: add engineering standards (dependabot)

[nikzen]

Automated rollout from engineering-standards.

Base (always included):

• .github/dependabot.yml – automated dependency updates (Dependabot)
• Ecosystems: github-actions, dart
• Grouped PRs: major updates separate from minor+patch
• Cooldown: 14 days

Detected scope: dart

[CLAassistant]

All committers have signed the CLA.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 57.55%. Comparing base (ba64888) to head (fe1cc41).
⚠️ Report is 4 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2305      +/-   ##
==========================================
+ Coverage   57.16%   57.55%   +0.38%     
==========================================
  Files         159      159              
  Lines       19778    19778              
==========================================
+ Hits        11307    11384      +77     
+ Misses       8471     8394      -77     

see 10 files with indirect coverage changes

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[krille-chan]

We must not remove the versioning-strategy for dart packages as we want to update the range for major versions only if necessary to let the consumer app in the end decide about the packages. So for packages we must use a different versioning-strategy than for apps in the dart ecosystem