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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[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.

[pierrejeambrun]

Thanks @pranampagi

1. Some of these change the rendered casing. Where the original text used a lowercase noun mid-sentence, nesting the title-case key capitalizes it:

• connections.testDisabled: "Test connection feature…" → "Test Connection feature…"
• pools.delete.warning: "the pool … this pool" → "the Pool … this Pool"
• pools.noPoolsFound: "No pools found" → "No Pools found"
• variables.import.uploadPlaceholder: "containing variables" → "containing Variables"
• variables.noRowsMessage: "No variables found" → "No Variables found"

Could we leave those running-text strings un-nested? The nesting is great for labels/buttons/titles (those are title-case already), just not for mid-sentence prose where the noun was lowercase on purpose. Or we accept this difference if we think it's fine.

2. Since this is a values-only change, could you add before/after screenshots of the Connections/Pools/Variables screens (or confirm you checked them)? Just to be sure none of the rendered strings shifted unintentionally.

[pranampagi]

Thanks for the review @pierrejeambrun

1. Casing fixed: Good catch. I've reverted all mid-sentence prose strings back to literal lowercase text. The $t() nesting is now strictly used for title-case contexts like buttons, table headers, and form labels.
2. UI verification: To ensure none of the rendered strings shifted unintentionally, I verified the resolution paths locally and took before/after screenshots. As shown below, the UI output is identical.

Pools Screen (Empty state & Delete warning)

Before

After

Variables Screen (Empty state & Import modal)

Before

After

Connections Screen (Test disabled alert & Delete modal)

Before

After

[pranampagi]

I've just pushed an update to resolve the 4 failing CI checks from the last run:

1. E2E Test Timeouts: Reverted the $t() nesting for specific ID strings (like Dag ID and Task ID) back to literal text. Playwright was timing out trying to assert the strings during the interpolation phase. This also aligns with the earlier feedback to avoid over-nesting basic text.
2. Linting Error: Fixed a missing trailing newline in common.json that was failing the check-i18n-json pre-commit hook.

[pranampagi]

Thanks for running the CI—it's great to see that the fixes worked and the build passed perfectly (38 successful checks).

I've just updated the branch to keep it perfectly in sync with main. Could you please trigger the CI checks one last time so we can get this merged?

Thanks again for the help!