Fix composer toolbar collapse flicker on tab switch (#2937)

Retained tabs stay mounted under display:none while inactive, which
reports a zero-width onLayout. ControlledAgentControls fed that
straight into its density resolver, so a hidden tab's toolbar
collapsed to its tightest density and painted one frame collapsed
before re-expanding when the tab became visible again.

Extract the zero-width guard that useContainerWidthBelow already had
into a shared readMeasuredWidth helper, and use it in the composer
toolbar's own layout handler so a display:none measurement can never
drive its density.

Also bump the chat outline rail's MIN_PANEL_WIDTH breakpoint (818 ->
918) so the rail disappears earlier under a narrower panel, and widen
the chat-outline e2e spec's viewport so its panel keeps healthy margin
over the new threshold.

packages/app/e2e/browser/chat-outline.spec.ts

@@ -34,7 +34,10 @@ import {
   type LongTimelineAgent,
 } from "../support/helpers/timeline-pagination";
 
-const WIDE_VIEWPORT = { width: 1280, height: 900 };
+// Wide enough that the timeline panel clears the rail's MIN_PANEL_WIDTH with room
+// to spare. At 1280 the panel measures 960, which sits too close to the threshold
+// for chrome-width changes elsewhere to stay out of these tests.
+const WIDE_VIEWPORT = { width: 1440, height: 900 };
 const LOADED_TURNS = 16;
 
 test.describe("desktop chat outline", () => {