feat(joint-react): set sensible Paper defaults and refine theme

[kumilingus]

Summary

Paper defaults via PresetPaper base class

• Extract all Paper defaults into a new PresetPaper class (dia.Paper.extend(...)) that PortalPaper now extends
• Required (not overridable): async, sorting, frozen, autoFreeze, viewManagement
• Overridable defaults: linkPinning=false, gridSize=10, clickThreshold=5, snapLinks={radius:15}, markAvailable=true, drawGrid=true, preventDefaultBlankAction=false, defaultConnectionPoint, measureNode
• Remove ~40 lines of default-merging logic from PaperStore constructor — it now just passes paperOptions through
• Inject --jr-paper-grid-color CSS custom property into all built-in grid patterns

Highlighting presets

• Move highlighting config from PaperStore to PresetPaper as DEFAULT_HIGHLIGHTING
• Default highlight uses CSS-driven stroke highlighter with --jr-paper-highlight-color
• Magnet/element availability now use addClass highlighter (jr-available-magnet, jr-available-element) instead of stroke — enables CSS-only theming

Theme CSS updates

• Link wrapper default width: 0 → 10 (clickable hit area)
• Link line: add stroke-linecap fallback ('butt'), remove stroke-linejoin (handled by markup)
• Port outline defaults: transparent/0 → #EFF2F5/2 (visible outline)
• Add .jr-port-body.jr-available-magnet rule with --jr-port-available-color
• Add .jr-available-element .jr-element rule with --jr-element-available-border

Portal link markup cleanup

• Remove redundant inline stroke: 'transparent' and strokeLinecap: 'round' from wrapper markup — now handled by CSS theme

Consolidate duplicated data mappers

• flatMapDataToElementAttributes / flatMapLinkAttributesToData (internal) now delegate to flatElementDataToAttributes / flatAttributesToElementData (public), eliminating ~150 lines of copy-pasted logic
• Internal wrappers add only id (forward) or default-value filtering (reverse)
• Fixes divergence: attrs.root.magnet: false for ports is now applied consistently (was missing in internal mapper)
• Remove stray debugger statement

Minor cleanup

• Rename createPortDefaults → createPortGroupsDefault
• import { dia, mvc } → import type { dia } in portal-paper (remove unused mvc)
• (keyof T)[] → Array<keyof T>, rename abbreviated variables in useFlatElementData

Storybook: dark theme demo

• Add dark theme toggle to with-default-element story
• New dark-theme.css with CSS variable overrides demonstrating full theme customization

Test plan

• Verify Paper renders with new defaults (grid visible, links not pinnable, click threshold feels responsive)
• Verify paperOptions overrides still take effect (e.g. gridSize={1})
• Verify port/element availability highlighting uses CSS classes
• Verify highlighting={false} still disables all highlighting
• Verify link wrapper is clickable (10px hit area)
• Verify port outlines are visible by default
• Verify elements with ports have attrs.root.magnet: false set consistently
• Verify dark theme toggle works in with-default-element storybook
• data-mapper.test.ts and use-flat-link-data.test.ts pass

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