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

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[boring-cyborg]

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[pranampagi]

Validation performed:

• Ran a local Node script utilizing Airflow's exact i18next configuration to verify resolution paths.
• Confirmed $t(common:dag_one) resolves to Dag across namespaces.
• Confirmed $t(connections.connection_one) resolves to Connection internally.
• Verified pnpm build completes successfully with the updated translation files.
• Verified pre-commit passes specifically for the modified .json files.