feat: Custom layer builder wizard UI (#66)

[deucebucket]

Parent: #66 Phase 5 - Custom Layers & Plugin System

What

A 4-step wizard in Settings → Plugins that lets users create custom API layers without code.

Wizard Steps

1. Basic Info - Name, description, icon picker
2. API Config - URL template with variable chips ({{title}}, {{author}}), HTTP method, auth type, API key
3. Response Mapping - Map JSONPath expressions to book fields (title, author, narrator, series, etc.), set confidence weight
4. Test & Preview - Enter a known book title/author, hit "Test API", see extracted results, save

Implementation

• New "Plugins" tab in settings.html (follows existing tab pattern)
• Multi-step form using Bootstrap d-none show/hide (same as setup_wizard.html)
• POST /api/plugins/test-layer endpoint - proxies the API call and returns mapped results
• POST /api/plugins/save-layer endpoint - saves to config.json custom_layers
• DELETE /api/plugins/layer/<id> endpoint - removes custom layer
• Variable chip UI showing available template variables
• JSONPath helper hints in response mapping step
• List of existing custom layers with edit/delete/enable-disable

Depends on

• Template HTTP layers backend (sibling issue)

Part of

#66 Phase 5, Tier 1

[deucebucket]

Implemented and merged to develop in commit 3a63024