test(shell): check 900x600, which is narrower than the minimum

The sidebar collapses to icons *below* 900, so the main panel is 843px
at 899 and 700px at 900: the narrowest content area any desktop width
produces is at the top of the Compact band, not at the enforced floor.
A viewport list that stopped at "the minimum" was missing its own worst
case.

MinWidth's comment loses both reasons it used to give, because neither
mechanism can happen any more — the subtitle is display:none from 899
down, and the sidebar host is overflow-y:auto (at 600x460 its
scrollHeight is 434 against a 332px client, and Settings is reachable
after scrolling). The value does not change: 800x600 is where desktop
chrome stops being comfortable, not where the app breaks, and below
600 the phone layout takes over. A floor defended by two expired
mechanisms is a number nobody can argue with, which is worse than
either answer.

Closes #24

e2e/specs/layout-overflow.spec.ts

@@ -26,6 +26,12 @@ const MIN_VIEWPORT = { width: 800, height: 600 };
 const VIEWPORTS = [
   { name: '1440×900', width: 1440, height: 900 },
   { name: '1024×768', width: 1024, height: 768 },
+  // Not the minimum, and that is the point (#24). The sidebar collapses
+  // to icons *below* 900, so the main panel is 843px at 899 and 700px
+  // at 900 — the narrowest content area any desktop width produces is
+  // here, not at the enforced floor. A list that stopped at the minimum
+  // was missing its own worst case.
+  { name: '900×600 (the widest sidebar, so the narrowest content)', width: 900, height: 600 },
   { name: `the minimum (${MIN_VIEWPORT.width}×${MIN_VIEWPORT.height})`, ...MIN_VIEWPORT },
 ];