fix: address PR #26 code review comments - memory, accessibility, and performance improvements

[Copilot]

Implements 17 code review suggestions from PR #26 focusing on memory management, accessibility, and performance optimizations for the media gallery enhancement.

Memory & Performance

Video frame extraction

• Cap canvas dimensions to 512×512 max (prevents unbounded memory usage on 4K+ videos)
• Implement LRU cache with 20-video limit and automatic eviction
• Fix timestamp calculation to avoid seeking to 100% (prevents codec failures):

  // Before: const timestamp = (duration / (frameCount - 1)) * currentFrameIndex;
  // After: Use 99% bounds
  const progress = currentFrameIndex / frameCount;
  timestamp = duration * progress * 0.99;

Media matching optimization

• Strip invisible Unicode characters once during map building (was: per-message in loop)
• Lower filename matching threshold from 10→8 chars (catches "document.pdf" without extension)

Accessibility

Keyboard navigation for video scrubbing

• Arrow Left/Right: Navigate between frames
• Enter/Space: Open video
• Added onfocus/onblur handlers to trigger frame loading

Screen reader support

• Added aria-label="Open {item.name}" to all media buttons
• Video-specific cursor (hover:cursor-ew-resize) indicates scrubbing capability

Bug Fixes

JSZip internal property access

// Added try-catch with fallback
const zipData = (zipEntry as any)._data;
if (zipData && typeof zipData.uncompressedSize === 'number') {
  size = zipData.uncompressedSize;
}

File extension handling

• Return empty string (not "FILE") when no extension exists
• UI shows generic document icon for extensionless files

Frame cache cleanup

• Call clearFrameCache() in component onDestroy (prevents memory leak on navigation)

Code Quality

• Added duplicate filename detection with console warnings
• Clarified SVG categorization comments (images vs documents context)
• Fixed event listener cleanup ({ once: true } where applicable)
• Improved file size formatting (rounded values, no decimals for bytes)

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