feat(design): C5 recorded, every E28 decision taken

The owner's lean Plex control set with both refinements (Back by position,
a touch double-tap for the 10 s skip) goes into DESIGN.md's Players
chapter and the log. The lab is now the reference only.

frontend/src/design-lab/DesignLab.tsx

@@ -3,10 +3,7 @@ import clsx from "clsx";
 import { SCHEMES, type Scheme } from "../lib/theme";
 import { BatchA } from "./batchA";
 import { BatchB } from "./batchB";
-import { BatchC, C5 } from "./batchC";
-
-// The open decisions, in the order they are asked. The decided ones render below them as reference.
-const OPEN = [{ id: "c5", code: "C5", title: "Player controls", render: C5 }] as const;
+import { BatchC } from "./batchC";
 
 // The design lab (E28): the living style guide. Dev-only — see main.tsx and devOnlyGuard.ts. It sits
 // OUTSIDE <App>, so it owns the theme attributes on <html> itself, through the same data-theme /
@@ -93,15 +90,6 @@ export default function DesignLab() {
           />
         </label>
         <nav className="flex flex-wrap gap-1 text-xs">
-          {OPEN.map((d) => (
-            <a
-              key={d.id}
-              href={`#${d.id}`}
-              className="rounded-md px-1.5 py-0.5 text-muted hover:bg-card hover:text-fg"
-            >
-              {d.code} {d.title}
-            </a>
-          ))}
           <a href="#decided" className="rounded-md px-1.5 py-0.5 text-success hover:bg-card">
             A ✓
           </a>
@@ -116,15 +104,12 @@ export default function DesignLab() {
 
       <main className="mx-auto flex max-w-screen-2xl flex-col gap-6 p-4">
         <header>
-          <h1 className="text-2xl font-bold">Open · the Plex player controls</h1>
+          <h1 className="text-2xl font-bold">The design reference</h1>
           <p className="text-sm text-muted">
-            Each decision shows its variants with the app&apos;s real CSS. The framed one is the
-            recommendation. Resize the window to tablet size to see them there too.
+            Every rule of docs/DESIGN.md in its chosen rendering, with the app&apos;s real CSS.
+            Resize the window to tablet size to see them there too.
           </p>
         </header>
-        {OPEN.map((d) => (
-          <d.render key={d.id} />
-        ))}
         <header id="decided" className="mt-6 scroll-mt-20">
           <h2 className="text-xl font-bold">Decided · batch A, controls</h2>
           <p className="text-sm text-muted">