UI: Update template editor for sequence/branch model

[b-at-neu]

Summary

Update the process template editor (/processes/[id]) to render and manage the new sequence/branch step model. Supersedes #597.

UI Behavior

Rendering

• Root sequence: render as a vertical list of step cards
• Branch sublists: for each step that has branches, render each branch as an indented vertical sublist below the step
• Each branch is visually distinct (e.g. labeled "Branch 1", "Branch 2", or by first step name)

Actions

• "Add Step": appends to the root sequence (or to the current sequence context)
• "Add Branch": opens a form that starts a new parallel sequence branching off the clicked step — no header/container step required
• Move up/down: reorders within the same sequence only
• Delete: re-links the sequence (predecessor → successor)

Local State

type LocalStepNode = {
  id: string
  name: string
  description: string | null
  previousStepId: string | null
  branches: LocalStepSequence[]
}
type LocalStepSequence = LocalStepNode[]

type AddFormState =
  | { type: 'root' }
  | { type: 'branch'; parentStepId: string }
  | null

Blocked By

Blocked by #613 (service layer)

Supersedes #597

Part of #530

[b-at-neu]

Blocked by #613