[Feature] Emit custom error events for host application observability

[numbers-official]

Summary

Currently, all error handling across the codebase uses bare console.error() calls with no mechanism for host applications to detect or respond to failures. When asset data fails to load, the modal shows an infinite loading shimmer with no error feedback, and embedding websites have no way to implement fallback UI, retry logic, or error monitoring.

Problem

1. Silent failures: fetchAsset() in src/asset/asset-service.ts returns undefined on network errors or non-OK responses (lines 15-19, 67-69). The modal continues showing shimmer placeholders indefinitely.
2. No host integration: Embedding websites cannot detect when Capture Eye fails — there are no events, callbacks, or observable states exposed.
3. Unobservable tracking errors: InteractionTracker.createEvent() in src/modal/interaction-tracker.ts (lines 86-88) logs errors to console but provides no external signal of failure.
4. No retry capability: Once a fetch fails, there is no way for the host application to trigger a retry without removing and re-adding the component.

Proposed Approach

1. Dispatch capture-eye-error CustomEvent from the <capture-eye> component

// In capture-eye.ts or modal-manager.ts, after a fetch failure:
this.dispatchEvent(new CustomEvent('capture-eye-error', {
  bubbles: true,
  composed: true,  // Cross shadow DOM boundary
  detail: {
    type: 'fetch-asset' | 'fetch-metadata' | 'event-tracking',
    nid: this.nid,
    message: error.message,
    timestamp: new Date().toISOString(),
  },
}));

2. Dispatch capture-eye-loaded success event

this.dispatchEvent(new CustomEvent('capture-eye-loaded', {
  bubbles: true,
  composed: true,
  detail: { nid: this.nid },
}));

3. Add an error state to the modal

Instead of infinite shimmer on fetch failure, show a minimal error state (e.g., "Unable to load asset data") with an optional retry action.

Affected Files

• src/asset/asset-service.ts — Error handlers at lines 15-19, 67-69, 85-88, 113-116
• src/modal/modal-manager.ts — Where fetchAsset results are consumed
• src/modal/modal.ts — Add error state rendering alongside shimmer/loaded states
• src/capture-eye.ts — Expose events to host applications

Expected Impact

• Host applications can implement error boundaries and fallback UI
• Production monitoring tools can track Capture Eye reliability
• Users see clear error feedback instead of infinite loading states
• Improved developer experience for integrators