Add missing UI translations for Greek(el)

[xBis7]

I ran breeze ui check-translation-completeness --language el to identify all the missing translations, used Claude to fix all the generated TODOs and then reviewed the changes and made some corrections.

Apart from the missing translations, I also added some extra terms in the el.MD file for agents, and made some corrections to issues that weren't flagged by the breeze command but I happened to notice while reviewing the files.

With this PR, there should be 100% coverage for the Greek language translation.

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Was generative AI tooling used to co-author this PR?

• Yes (please specify the tool below)
  Claude code Opus 5

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[shahar1]

I let GPT to review the translation - please address its 3 concerns and let's get this merged (if any of the comments is wrong, feel free to resolve).
Thank you!

[github-actions]

Hi maintainer, this PR was merged without a milestone set.
We've automatically set the milestone to Airflow 3.3.1 based on: backport label targeting v3-3-test
If this milestone is not correct, please update it to the appropriate milestone.

This comment was generated by Milestone Tag Assistant.

[github-actions]

Backport successfully created: v3-3-test

Note: As of Merging PRs targeted for Airflow 3.X
the committer who merges the PR is responsible for backporting the PRs that are bug fixes (generally speaking) to the maintenance branches.

In matter of doubt please ask in #release-management Slack channel.

Status	Branch	Result
✅	v3-3-test

[xBis7]

@shahar1 Thank you!