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

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Generated-by: OpenAI Codex following the guidelines

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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.

[boring-cyborg]

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[potiuk]

@chocolatecake777 A few things need addressing before review — see our Pull Request quality criteria.

• ❌ CI fails: Basic tests / React UI tests (and possibly other checks — see the Checks tab for the full list).

No rush.

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Note: This comment was drafted by an AI-assisted triage tool and may contain mistakes. Once you have addressed the points above, an Apache Airflow maintainer — a real person — will take the next look at your PR. We use this two-stage triage process so that our maintainers' limited time is spent where it matters most: the conversation with you.

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Drafted-by: Claude Code (Opus 4.7); reviewed by @potiuk before posting

[choo121600]

@chocolatecake777 Thanks for the contribution!
This issue has already been resolved in #67169, so I'll close this PR.
We’d love to see more contributions from you in the future ;)