fix(studio): keep the preview alive when the window is tight (#3091)

Panel sizes are reconciled against the window on every resize, with the preview holding a 360x200 floor that panels yield to before it gives.

Measured preview pane: 760px window 192 -> 433, 560px window 2 -> 516. Windows at or above 1280px are unchanged.

- fitPanels owns the who-yields decision for both axes
- panel caps are window-relative, replacing a flat 600px inspector cap
- below 860 the sidebar rails, below 700 the inspector collapses too
- auto-collapse is derived render state and never writes leftCollapsed
  (localStorage) or rightCollapsed (synced into the shareable URL)
- the rail and header toggles act on the effective state, so neither is a
  dead click that silently persists a collapse the user never asked for

packages/studio/src/components/StudioHeader.test.ts

@@ -0,0 +1,28 @@
+import { describe, expect, it } from "vitest";
+import { shouldOpenInspector } from "./StudioHeader";
+
+describe("shouldOpenInspector", () => {
+  it("opens when the panel is hidden", () => {
+    expect(shouldOpenInspector(true, false)).toBe(true);
+  });
+
+  it("opens when a non-inspector tab is showing", () => {
+    expect(shouldOpenInspector(false, false)).toBe(true);
+  });
+
+  it("closes when the inspector is genuinely on screen", () => {
+    expect(shouldOpenInspector(false, true)).toBe(false);
+  });
+
+  it("opens when the window railed the panel away", () => {
+    // The regression this guards: the button used to branch on the raw
+    // rightCollapsed intent, which is still `false` while the window has the
+    // panel railed. That took the close branch, wrote rightCollapsed=true, and
+    // since that value is synced into the shareable Studio URL, a click that
+    // did nothing visible rewrote the link.
+    const userIntentIsOpen = false;
+    const windowRailedItAway = true;
+    expect(shouldOpenInspector(windowRailedItAway, true)).toBe(true);
+    expect(shouldOpenInspector(userIntentIsOpen, true)).toBe(false);
+  });
+});