[configuration] Show nested configuration settings

[Montekkundan]

Brief summary of changes

• Builds configuration settings as a recursive tree under each configuration category.
• Replaces the unfinished nested-setting placeholder with collapsible nested UI blocks.
• Keeps existing leaf setting controls and submit behavior unchanged.

Note: This PR implements the tree/nested UI part of #10449 for #10471. Mapping field support is handled separately in #10458, and category separation is handled in #10457.

This PR is stacked on top of the config categories work from #10457. The #10471 specific change is the nested/tree UI rendering on top of that base.

Testing instructions

• Open http://localhost:8080/configuration/ as a user with config permission.
• Confirm the existing category tabs and settings still render.
• Add or use a nested ConfigSettings.Parent test setting and confirm the parent group appears as a collapsible block.
• Expand the group and confirm child settings render with the existing input controls.
• If testing with a text child setting, confirm it renders as a text input. If testing with a boolean child setting, confirm it renders with the existing Yes/No controls.

Link(s) to related issue(s)

• Resolves [configuration] Rework to support multidimensional UI #10471