Wire Add Subtopics chip to suggest-then-create flow with inline UI

[davstur]

Overview

Wire the "Add subtopics" action chip in CollapsibleSection to the suggest and create subtopics APIs. When tapped, show AI-suggested subtopics, let the user select/customize, then generate and display new child nodes inline.

User Story

User Story: /docs/user-stories/active/create-subtopics.md

As a user, I want to add child sections to any node, so I can branch out and explore adjacent areas within a topic.

Suggested Priority: high 🟠

Reasoning: Turns the last "Coming soon" chip into a real feature, completing the Explorer's action chip set.

Requirements

• Tapping "Add subtopics" chip opens SubtopicPanel inline
• Panel shows loading while fetching suggestions
• 3 AI suggestions displayed as selectable chips (label + emoji)
• User can select multiple suggestions (toggle on/off)
• Freetext input for custom subtopic labels
• "Create" button generates content for selected + custom labels
• Loading state during creation
• New child nodes appear in the tree below the parent
• Error state with retry
• Not available on H4 nodes (max depth — chip already hidden)
• All interactive elements have testID props

Technical Approach

UI Flow

Tap "Add subtopics" chip
  → SubtopicPanel opens inline
  → Loading: fetching suggestions...
  → 3 suggestion chips appear (toggleable)
  → User selects suggestions + optionally types custom labels
  → Tap "Create" button
  → Loading: generating content...
  → New child nodes appear in tree
  → Panel closes

Components

• SubtopicPanel: inline below chips (like ExpandPanel pattern)
• SuggestionChip: toggleable chip showing emoji + label
• Custom label input with "+" button

State Management

• On successful creation: add new child nodes to parent in tree state
• Update the parent's children array + recalculate childCount
• New nodes should inherit branchColor from parent's branch

Definition of Done

• Suggest + create flow works end-to-end
• AI suggestions are relevant and selectable
• Custom labels work alongside suggestions
• New nodes render correctly in the tree
• Loading and error states work
• testID props on all elements

Related

• User Story: /docs/user-stories/active/create-subtopics.md
• Depends on: Implement SUGGEST_SUBTOPICS and CREATE_SUBTOPICS API endpoints #24 (suggest + create endpoints)
• CollapsibleSection: components/CollapsibleSection/CollapsibleSection.tsx
• Explorer: app/topic/[id].tsx
• ExpandPanel pattern: same component, parallel flow

[davstur]

🤖 Claude Code has started implementing this issue on branch feature/24-25-create-subtopics (combined with #24)