Fix UI Skills violations across frontend components

Commit notes

Accessibility fixes: - Add aria-label to icon-only buttons in SequencerTrack, Header, library page - Add aria-label and role=checkbox to sample selection buttons - Replace native confirm() with accessible AlertDialog component

Animation/Performance fixes: - Replace transition-all with specific properties (transform, opacity, etc.) - Fix animation durations exceeding 200ms (300ms -> 200ms) - Use transition-[width] instead of transition-all for progress bars

Typography fixes: - Add text-balance to headings - Add text-pretty to body paragraphs

Layout fixes: - Replace arbitrary z-60/z-61 with semantic z-modal-backdrop/z-modal utilities - Replace w-* h-* with size-* for square elements - Add z-index utilities to app.css

New component: - AlertDialog.svelte for destructive action confirmations

Co-authored-by: armin.naimi <[email redacted]>