Improve HTML report triage UX, filters, and attachment diagnostics

[oscarcv]

Summary

This MR upgrades the default HTML report for large snapshot suites with better triage ergonomics and clearer diagnostics.

UX / Navigation

• Added top toolbar with:
  • search by normalized test title
  • status filters (All, Failed, Passed, Skipped)
  • failed-first sorting toggle
  • expand/collapse controls
• Added suite-level collapse/expand controls.
• Default behavior keeps suites collapsed (triage-friendly).
• When filtering/searching is active, visible suites auto-expand.

Variant Filtering

• Added multi-select variant filters:
  • High Contrast Dark
  • Dark
  • High Contrast Light
  • Light
• Variant selection is additive (toggle chips), not single-select.

Persistence & Accessibility

• Persisted UI state via localStorage:
  • search query
  • status filter
  • selected variants
  • failed-first toggle
  • manual suite collapse state
• Added keyboard shortcuts:
  • / focus search
  • f failed filter
  • e expand/collapse
• Improved focus visibility and toolbar polish.

Attachment Diagnostics

• Added attachment metadata in context/rendering:
  • exists
  • isEmpty
  • sizeBytes
• UI now shows explicit diagnostics for:
  • missing attachment files
  • empty attachment files
• Added file size metadata display in attachment cards.

Files Changed

• Sources/SnapshotReportCore/Resources/default-report.stencil
• Sources/SnapshotReportCore/Reporters/HTML/HTMLSnapshotReporter.swift
• Tests/SnapshotReportCoreTests/SnapshotReportCoreTests.swift

Validation

• swift test (all tests passing)
• End-to-end HTML generation from example xcresult:
  • snapshot-report --xcresult ... --format html --verbose
• Verified generated HTML includes:
  • multi-select variant filters
  • title-based search
  • filtering-driven auto-expand
  • attachment empty/missing diagnostics