docs: add UI/UX component guidelines and improve breadcrumb/tab-panel styling
Add "Komponenten in Formularen" section to boundaries-ui.md with rules
for core button classes, margin resets, checkbox sizing, sticky positioning,
icon-only buttons, responsive grids, and CSS variables. Add matching
quick-reference bullets to CLAUDE.md UI Patterns section.
Fix breadcrumb wrapping: nowrap + overflow hidden + flex-shrink for
single-line behavior. Add data-tab-panel-wide opt-out for full-width
tab panels (default 70ch).
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
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