Size the rail to the traffic lights instead of guessing

The collapsed rail was a fixed 72px, which left the cluster with more
room on its left than its right. The width now comes from the buttons
themselves - the close button's own inset plus the zoom button's right
edge - so the margin matches on both sides. That inset is macOS's to
choose and has changed between releases, so it is asked for rather than
assumed.

Measured at startup rather than on demand. The answer arrives on the main
thread, and a command waiting for it there deadlocks until the timeout
and silently returns the fallback - which is what the first attempt did.

docs/superpowers/specs/2026-09-01-work-app-design.md

@@ -150,7 +150,11 @@ ladder, and light and dark both designed rather than one derived from the other.
 - **Nav, expanded (~240px)** — traffic-light drag inset, title with cog and collapse
   chevron, then groups as collapsible sections with uppercase tracked labels, apps as
   favicon-and-name rows. The active row inverts to `bg-slate-900 text-white`.
-- **Nav, collapsed (~52px)** — favicons only, active marked with a left accent bar, name on
+- **Nav, collapsed** — as wide as the traffic lights need, which is asked of AppKit
+  rather than guessed: the rail is the close button's own left inset plus the zoom
+  button's right edge, so the cluster keeps the same margin on both sides. That inset is
+  macOS's to choose and has changed between releases.
+- **Nav, collapsed (old note)** — favicons only, active marked with a left accent bar, name on
   hover, hairline dividers between groups. Still clickable, so switching does not require
   expanding.
 - **Top bar (~38px)** — back, forward, reload, the current URL muted and truncated, and