[6.x] Forms 2: Logic Tree

[jaygeorge]

This pull request implements a tree view for the form Logic tab — a visual, column-per-page overview of a form's pages, sections and fields, with drag-and-drop reordering and inline editing of each field's (or page's) logic.

This PR also refactors the Logic page to share the form builder's nested form-fields data model, so both tabs serialise fields through a single path.

Tree view

A new List / Tree toggle sits on the Logic tab (the chosen view is remembered per user).

The tree renders one column per page, with:

• Sections shown as labelled groups, each with its own field list
• Field rows with fieldtype icons, plus reference/imported-field indicators
• Conditional connectors drawn from a field to its destination page (including multi-page "leaps"), using CSS anchor positioning
• An Expanded / Collapsed density toggle (matching the Edit tab)

Fieldset imports appear as a single grouped node, mirroring how they're treated in the form builder.

Editing logic

Clicking a field or page opens a resizable panel beneath the tree (using the Splitter component) to edit its logic without leaving the view:

• Field → its show/hide conditions
• Page → its routing rules

Pressing escape deselects. The final page and imported fields aren't selectable, since they can't hold logic.

Reordering

Fields — and whole fieldset imports — can be dragged to reorder them within a section, between sections, and across pages. The new order is persisted on save.

Shared data model

Previously the Logic page kept its own flattened representation of a form's fields and a bespoke merge on save. This PR reworks it to hold the same nested formFields structure as the form builder and round-trip it through the shared FormFieldTransformer.

The toVue / fromVue conversion now lives in the ManagesFormFields trait, used by both the builder and logic controllers. As a result FormLogicController sheds its bespoke serialisation/merge code, and reordering, imported fields and referenced-field logic (stored under config) all round-trip consistently between the two tabs.

Screenshot

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Related: #14909

[duncanmcclean]

Thanks, Jay!

I've refactored some things behind the scenes to make things more consistent w/ the form builder, added the ability to reorder fields and utilised the Splitter component for editing logic.

When you're back, you might want to tweak the design of the splitter/logic box. I just went with what Claude generated but not sure that's necessarily what we want. Up to you! 😄