feat(ui): quiet the layer chrome and tighten layer rows

[winetree94]

What

Every popup layer — the menubar menus, session and workspace dropdowns, the terminal context menu, select popups, composer suggestion layers — framed itself with borderStrong and stacked rows at a taller density than the select options beside them. This re-pins tinyrack_ui to ce4ae39b (design#443), which:

• lowers every layer's border from borderStrong (#737373 / #a3a3a3) to border (#d4d4d4 / #404040), keeping the 1px token width;
• brings menu rows down from 32 to the 28px sm control height, so they match select options — layerComponents.menuItemHeight and optionItemHeight are both 1.75rem now;
• widens the TRMenuSeparator gap from spacing.xs to spacing.sm, so a divider reads as a group boundary rather than another row gap.

That release also fixes two defects it surfaced: a TRMenubar trigger drew no focus ring at all, and the TRTooltip border took Flutter's default width instead of the token.

One app-side change: the File menu built its divider from TRSeparator, the page-level rule, which never picks up the new group-boundary gap. It uses TRMenuSeparator now.

Verification

• dart run melos verify — 12/12 PASS
• xvfb-run -a dart run melos verify:debug — PASS, real Linux Debug runner and embedded daemon
• 8 Linux goldens regenerated and reviewed by eye: lighter frame, shorter rows, real space around the File-menu divider, focus ring now visible on the open menubar trigger. Nothing else moved.