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

backend/config/window.go

@@ -13,13 +13,31 @@ const (
 	// enforces this at runtime; it is also the floor below which a
 	// reported size is treated as bogus and not persisted.
 	//
-	// 800x600 is where the shell was measured to still work, rather
-	// than a round number: below ~780 the header's subtitle wraps and
-	// pushes the title out of the 4em top bar, and below ~600 tall the
-	// eleven sidebar items no longer fit at once.  The previous
-	// 512x384 was aspirational — at 700x480 the sidebar overflowed
-	// behind the player bar with no scroll and Settings and Jobs could
-	// not be reached at all.
+	// **Both reasons this comment used to give have expired**, and the
+	// value is right for a third one.  It said the floor was 800x600
+	// because "below ~780 the header's subtitle wraps and pushes the
+	// title out of the 4em top bar" and "below ~600 tall the eleven
+	// sidebar items no longer fit at once".  Neither mechanism can
+	// happen now: the subtitle is display:none from 899px down
+	// (index.css), and the sidebar host is overflow-y:auto — measured
+	// at 600x460, its scrollHeight is 434 against a 332px client and
+	// Settings is reachable after scrolling.  A floor defended by two
+	// mechanisms that no longer exist is a number nobody can argue
+	// with, which is worse than either answer.
+	//
+	// It stays 800x600 because that is where the *desktop* chrome
+	// stops being comfortable — the Compact band of plan 018's size
+	// matrix (#24) — and not because the app breaks below it.  It does
+	// not: under 600px wide the phone layout takes over (bottom-nav,
+	// no sidebar) and the shell fits 320px exactly, which is what
+	// makes this a comfort floor rather than a correctness one, and
+	// why a very small window reflows instead of becoming a
+	// mini-player (#12 is a second always-on-top window, not a mode of
+	// this one).
+	//
+	// The previous 512x384 was aspirational — at 700x480 the sidebar
+	// overflowed behind the player bar with no scroll and Settings and
+	// Jobs could not be reached at all.
 	MinWidth = 800
 	// MinHeight is the smallest allowed window height in pixels.
 	MinHeight = 600