UI: Group and filter Dags by folder on the Dags page

[RabahAmrouche05]

What

Adds a folder navigation tree to the Dags page. The tree is built from each
Dag's relative_fileloc directory, and selecting a folder filters the Dag list
to the Dags that live in it (and its subfolders).

Why

The Dags page only offered a flat list. In deployments with many Dags spread
across subfolders of the dags folder (e.g. team_a/etl/…, team_b/ml/…) there
was no way to browse or filter by folder.

How

• Backend — a server-side relative_fileloc_prefix filter on GET /ui/dags
  (strict folder match, so team_a never catches team_alpha) and a new
  GET /ui/dags/folders endpoint returning the distinct folders of all readable
  Dags. Filtering is done server-side so the result stays correct across
  pagination, sorting and the other filters.
• Frontend — a collapsible folder tree on the Dags page that drives the
  filter via a dag_folder search param, with the tree hierarchy reconstructed
  client-side from the folder paths.

Dags located at the bundle root contribute no folder and appear under "All Dags".

Tests: backend (folders endpoint + filter, incl. the team_a / team_alpha
edge case) and frontend (buildFolderTree unit tests + DagFolderTree
component tests).

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

• Yes — Claude Code (Opus 4.8)

Generated-by: Claude Code (Opus 4.8) following the guidelines

[boring-cyborg]

Congratulations on your first Pull Request and welcome to the Apache Airflow community! If you have any issues or are unsure about any anything please check our Contributors' Guide
Here are some useful points:

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[jscheffl]

Newsfragment is only added for important changes. I think this is not needed here.

[jscheffl]

Can you please add a screenshow or small video recording of how it looks like?