feat(sdk): the Properties header shows the selected node

[jimmeryn]

What

• The header shows the selected node's icon in its accent, its label and its type label. A node without a label shows its type label alone. A selected edge shows its icon (CaretRight by default), its label and "Link". headerLabel shows only while nothing is selected.
• The collapse toggle moves to the right. Label and subtitle are single lines with an ellipsis and a native title; the DS Tooltip takes over later (follow-up: node-text-ds-tooltip).
• The panel is a region named by headerLabel.
• The collapsed panel no longer grows wider than the open one with a long label.
• The node subtitle on the canvas no longer crops descenders (g, p, y).
• Changesets: sdk minor, sdk patch, ui patch.

Merge after #187

#187 (draft) reworks the same header: it adds PropertiesPanelHeaderContent (a row under the title), the properties panel menu items and the toggle actions, deprecates onMenuHeaderClick and renames toggleSidebar. This PR is rebased onto main once #187 merges, and the new heading is checked next to the header content area and the menu.

Part 2 of 2 in a stack: #203 -> this PR. Review in order.