Refactor UI translation files to use nested keys for DRYness

[pranampagi]

This PR modifies the Airflow UI translation files in the English (en) locale to utilize the $t() function for nested text references.

What and Why:
Previously, core concepts (like "Dag", "Task", "Connection") were repeated verbatim across hundreds of keys. This creates a large maintenance surface area and leaves room for translation inconsistencies across languages.
To make the localization files more DRY (Don't Repeat Yourself), this PR:

1. Establishes base concepts in common.json.
2. Implements internal nesting within common.json and admin.json (e.g., "$t(connections.connection_one)").
3. Implements cross-namespace nesting in dags.json (e.g., "$t(common:dag_one)") to prove that base concepts can be centralized and accessed from separate JSON files.

Closes: #68252

---

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

• Yes (please specify the tool below)

---

• Read the Pull Request Guidelines for more information. Note: commit author/co-author name and email in commits become permanently public when merged.
• For fundamental code changes, an Airflow Improvement Proposal (AIP) is needed.
• When adding dependency, check compliance with the ASF 3rd Party License Policy.
• For significant user-facing changes create newsfragment: {pr_number}.significant.rst, in airflow-core/newsfragments. You can add this file in a follow-up commit after the PR is created so you know the PR number.

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contributors' Guide
Here are some useful points:

• Pay attention to the quality of your code (ruff, mypy and type annotations). Our prek-hooks will help you with that.
• In case of a new feature add useful documentation (in docstrings or in docs/ directory). Adding a new operator? Check this short guide Consider adding an example Dag that shows how users should use it.
• Consider using Breeze environment for testing locally, it's a heavy docker but it ships with a working Airflow and a lot of integrations.
• Be patient and persistent. It might take some time to get a review or get the final approval from Committers.
• Please follow ASF Code of Conduct for all communication including (but not limited to) comments on Pull Requests, Mailing list and Slack.
• Be sure to read the Airflow Coding style.
• Always keep your Pull Requests rebased, otherwise your build might fail due to changes not related to your commits.
  Apache Airflow is a community-driven project and together we are making it better 🚀.
  In case of doubts contact the developers at:
  Mailing List: dev@airflow.apache.org
  Slack: https://s.apache.org/airflow-slack