Fix DAG browser tab title in React UI

[chocolatecake777]

Fix DAG pages in the React UI so the browser tab title reflects the selected DAG again instead of always remaining the generic Airflow title.

closes: #67145

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Summary

• Add a shared document-title hook that composes page-specific titles with the Airflow instance name.
• Use the hook from the base layout and DAG page so DAG views update the browser tab to the DAG display name.
• Add focused React tests covering DAG-specific document title behavior.

Verification

• Focused Vitest command for airflow-core/src/airflow/ui/src/pages/Dag/Dag.test.tsx passed in an isolated writable rerun after the initial read-only repository mount prevented test output writes.
• git diff --check HEAD~1 HEAD passed.
• Final branch status is clean at 82063a39ee019210f9f8441b377a9381080192eb.

Reviewer Notes

• The change is scoped to browser document-title handling in the React UI and does not change routing or DAG data loading behavior.

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