Single-source the heading type scale as CSS tokens #3046

Context

The heading scale (font-size, line-height, and the derived --icon-vertical-position) is hardcoded in two places: packages/desktop-app/src/lib/design/components/header-node.svelte and node-row.svelte (the latter repeats it for chevron/icon alignment). A third copy in a dead component already drifted out of sync once.

DESIGN.md names the tokens heading-1 … heading-4 (H4–H6 share heading-4).

Scope

• Define --heading-{1..4}-size / --heading-{1..4}-lh once in app.css
• Have header-node.svelte and node-row.svelte consume them; derive --icon-vertical-position from the tokens instead of literals
• No visual change (values identical to current)