fix: menus open like the other popovers, themed disclosures and a quiet sidebar scrollbar (0.4.2)

[AmrMsCLL]

Findings from a UI audit of Strata, released as 0.4.2.

Menus

• Menu now uses the same anchored top-layer panel as Select. Before, it was absolutely positioned under its trigger, so long menus ran off the screen and could not scroll. Strata's "Add widget" menu ended about 190px below the window.
• Menus align to the trigger's right edge through a new align: 'end' option on useAnchoredPanel. They flip upwards when there is no room below and scroll when taller than the window. The phone-only fixed position is gone, because anchoring covers it.
• Opening with a pointer focuses the menu itself, so no item is highlighted. Keyboard opening still focuses the first item, and the arrow keys start from the first item either way.

Disclosures

• <details>/<summary> use the interface's chevron instead of the browser's triangle and rotate it when open.

Sidebar scrollbar

• The sidebar shows its scrollbar only while hovered or focused. On shorter windows its content can overflow by a few pixels, and the permanent scrollbar looked like a fault.

Version 0.4.2 (package, lock file and the README install line). Tag v0.4.2 on the merge commit to publish.

Tests: new case in test/popovers.spec.tsx (pointer open, no highlighted item). Ran test/popovers.spec.tsx, test/workspace.spec.tsx, test/controls.spec.tsx and test/shell.spec.tsx, plus lint and build. Checked in Strata at 1440px and 390px: "Add widget", every widget menu, a space's row menu and the account menu all stay on screen.