fix(web): reconcile graph styles with dark mode

[ktjn]

Intent

Restore the graph hierarchy and contrast that were lost when the playground redesign removed the semantic graph-node CSS and dark mode later themed only React Flow's outer wrappers.

Changes

• add graph-specific light, system-dark, and explicit-dark theme tokens
• restore compact semantic node cards, kind badges, type accents, dashed version nodes, and visible focus/selection outlines
• improve canvas dots, edge and handle contrast, controls, and MiniMap treatment in both themes
• add a Playwright regression covering light/dark node styling and focus contrast
• document the approved design and implementation plan

Verification

• uv run ruff format . — passed, 274 files unchanged
• uv run ruff check . — passed
• mypy baseline ratchet — passed with 404 current errors and 45 resolved baseline errors
• uv run pytest --tb=short — 1,531 passed, 15 skipped
• npm test — 357 passed
• npm run check — passed
• npm run build — passed
• npx playwright test tests/playground.spec.ts — 20 passed
• manually inspected Domain, Entity, Version, Projection, and Field nodes in explicit light and dark themes
• Doc/spec review: all phases passed

Risk

Low. The implementation is CSS-only and does not change graph data, layout, interactions, compiler behavior, or generated artifacts. The browser regression locks the key cascade and theme states.

Deferred work

The pre-existing SVG exporter currently cannot locate an SVG ancestor from React Flow's div viewport, so Export SVG produces no Blob. This PR intentionally does not broaden into export behavior.