feat(frontend): apply UI skills best practices

Commit notes

- Replace confirm() with AlertDialog in dashboard for Mollie disconnect - Fix transition-all with specific properties across 23 occurrences - Replace z-60 with z-modal-backdrop semantic utility in FullscreenLoader - Fix duration-300+ to duration-200 for interaction feedback - Add text-balance to headings for better typography - Add text-pretty to body paragraphs - Replace h-* w-* with size-* for square elements - Add aria-labels to icon-only buttons for accessibility

Files modified: - 29 frontend components and pages

Follows UI Skills guidelines from .cursor/skills/ui-skills/SKILL.md

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