Add LLM-driven nested grouping and ordered parts rendering

[elasticsounds]

Summary

• Structured page sectioning: Replace flat part_ids with a parts union schema (string | PartGroup) so the LLM can output nested groups directly (e.g., option_group > option > text + image), eliminating the heuristic structureActivityParts() post-processing
• Ordered parts as canonical representation: Use ordered_parts as the single source of truth for rendering, removing redundant flat texts, images, groups arrays from the LLM rendering context. All 16 prompt templates updated to consume ordered_parts with nested group support
• Preserve nested group structure: Introduce NestedGroupPart in web rendering to carry group structure through to the LLM renderer instead of flattening it
• Individual text entry IDs in sectioning: Expose per-text-entry IDs (e.g., pg014_gp009_tx001) to the sectioning LLM so it can pair specific texts with images in structured groups
• UI improvements: Add JSON inspection tab to section data panel, replace separate Text Groups/Images panels in storyboard overview with a unified ordered Content view showing nested group structure

Test plan

• All 929 tests pass
• TypeScript typecheck clean
• i18n extraction — 0 missing translations
• Manual: run sectioning on an activity page (e.g., sorting with image+text options) and verify the JSON tab shows nested groups
• Manual: verify storyboard overview shows parts in document order with nested group indentation