Show backfill Dag runs in the UI

[shivaam]

Use the backfill Dag-run entries API added in #67381 to make the Dag runs associated with a backfill inspectable in the UI.

What changed

• Add a backfill detail route with a paginated Dag runs table for created runs and skipped slots.
• Add a Details tab for the backfill configuration and metadata.
• Link to the detail screen from the Dag Backfills table and the active-backfill banner.
• Add Dag > Backfills > Backfill #N breadcrumb navigation.
• Translate the known skipped-slot reasons.
• Keep the existing pause, unpause, and cancel behavior available in the detail-page header.
• Refresh the backfill metadata, visible Dag-run page, and DAG backfill list after those actions.
• Remove the progress UI that fetched every page of Dag-run associations.

Request behavior

Opening the dedicated screen makes two backfill-specific requests:

1. Fetch the backfill metadata.
2. Fetch the currently visible page of associated Dag runs.

While the backfill is active and unpaused, both requests follow the configured auto-refresh interval so the duration, completion state, and visible Dag-run statuses stay current. Refreshing stops when the backfill is paused or completed. Changing pages requests only the selected page; the UI never sweeps hidden pages and does not require a new aggregation endpoint.

Pause, unpause, and cancel retain their existing behavior. After a successful action, the affected backfill views are refreshed immediately so they do not display stale state.

Related: #67381

Testing

• pnpm lint
• pnpm test — 95 test files, 696 tests passed
• pnpm build
• breeze testing ui-e2e-tests --browser chromium --test-pattern backfill.spec.ts -- --workers=1 — 8 tests passed
• Airflow commit hooks, including UI compile/format/lint and i18n validation

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

• Yes — Codex (GPT-5)

Generated-by: Codex (GPT-5) following the guidelines

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Drafted-by: Codex (GPT-5) (no human review before posting)