chore(release): redesign UI tokens and publish styles 0.8.26, ui 0.8.86

Visual-only redesign ("instrument panel"): cool graphite neutrals for light
and dark, layered hairline elevation, larger radii, a mono token for
eyebrows/data, keycap-style solid and secondary buttons, recessed input
wells and tab tracks, a stronger dialog, and a theme-driven focus outline.
No props, outputs or behaviour change.

Also records the already-published csr 0.8.42, styles 0.8.25 and ui 0.8.85
changes (optional chaining/nullish in the reactive runtime, shared
theme/accent cookie sync, BackToTop hidden attribute).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ui/test/account-shell-regressions.test.ts

@@ -8,6 +8,20 @@ import { Window } from "happy-dom";
 const source = (name: string) => readFileSync(uiComponentPath(name), "utf8");
 
 describe("account shell UI regressions", () => {
+  test("BackToTop is hidden in server HTML unless explicitly always visible", async () => {
+    for (const alwaysVisible of [false, true]) {
+      const html = await renderComponent(source("BackToTop"), { alwaysVisible });
+      const server = new Window();
+      try {
+        server.document.body.innerHTML = html;
+        expect(server.document.querySelector(".wrn-back-to-top")?.hasAttribute("hidden")).toBe(
+          !alwaysVisible,
+        );
+      } finally {
+        await server.happyDOM.close();
+      }
+    }
+  });
   test("Drawer has a self-contained close icon and a compact centered mobile header", async () => {
     const dom = mountHtml(
       await renderComponent(source("Drawer"), { title: "Account center", open: true, duration: 0 }),