Make the light screenshot actually light

The light inbox screenshot in the README was not light, and had not been
since it was first taken -- the pair showed the dark theme twice.

The app was never at fault: update() calls applyTheme() synchronously
and the CSS flips --bg to #f6f8fa as it should. The capture was.
Swapping the theme under setDeviceMetricsOverride produces a mixed
frame, the panes that re-rendered in the new theme and the rest of the
chrome still in the old one, while the DOM and computed styles insist
the whole page is light. Clicking the app's own toggle, setting the
attribute, pinning it against applyTheme with a MutationObserver,
installing that pin before the document loads, nudging the viewport and
forcing a full reflow all left the frame mixed.

Chrome launched at --window-size, with the emulation layer never
touched, renders it correctly. That is docs/screenshots-light.mjs, and
inbox-light.jpg is now genuinely light.

assertTheme() stays: without it the script wrote a dark screenshot under
a light caption and reported success, which is how this survived
unnoticed. The header says which shots are taken elsewhere and why.

docs/screenshots-light.mjs

@@ -0,0 +1,75 @@
+/**
+ * The light inbox shot, with no Emulation.setDeviceMetricsOverride at all --
+ * the window is simply launched at the size we want. The emulation layer is the
+ * prime suspect for the mixed-theme frames every other approach produced.
+ */
+import { spawn } from "node:child_process";
+import { writeFile } from "node:fs/promises";
+import { setTimeout as sleep } from "node:timers/promises";
+
+const OUT = process.argv[2] ?? ".";
+const PORT = 9334;
+const chrome = spawn("google-chrome-stable", [
+  "--headless=new", `--remote-debugging-port=${PORT}`, "--hide-scrollbars",
+  "--no-first-run", "--no-default-browser-check",
+  "--window-size=1420,790", "--force-device-scale-factor=1",
+  "--user-data-dir=/tmp/claude-light-profile", "about:blank",
+], { stdio: "ignore" });
+
+const json = async (p) => { for (let i = 0; i < 60; i++) { try { return await (await fetch(`http://127.0.0.1:${PORT}${p}`)).json(); } catch { await sleep(250); } } throw new Error("no chrome"); };
+const version = await json("/json/version");
+let id = 1; const pending = new Map();
+const ws = new WebSocket(version.webSocketDebuggerUrl);
+await new Promise((r, j) => { ws.onopen = r; ws.onerror = j; });
+ws.onmessage = (m) => { const x = JSON.parse(m.data); if (x.id && pending.has(x.id)) { const { resolve, reject } = pending.get(x.id); pending.delete(x.id); x.error ? reject(new Error(JSON.stringify(x.error))) : resolve(x.result); } };
+const send = (method, params = {}, sessionId) => new Promise((resolve, reject) => { const i = id++; pending.set(i, { resolve, reject }); ws.send(JSON.stringify({ id: i, method, params, ...(sessionId ? { sessionId } : {}) })); });
+
+const { targetId } = await send("Target.createTarget", { url: "about:blank" });
+const { sessionId } = await send("Target.attachToTarget", { targetId, flatten: true });
+const cmd = (m, p) => send(m, p, sessionId);
+await cmd("Page.enable"); await cmd("Runtime.enable");
+const evaluate = async (expression) => {
+  const r = await cmd("Runtime.evaluate", { expression, awaitPromise: true, returnByValue: true });
+  if (r.exceptionDetails) throw new Error(r.exceptionDetails.exception?.description ?? "eval failed");
+  return r.result.value;
+};
+const waitFor = async (expr, what, ms = 20000) => {
+  const end = Date.now() + ms;
+  while (Date.now() < end) { if (await evaluate(`!!(${expr})`)) return; await sleep(200); }
+  throw new Error(`timed out waiting for ${what}`);
+};
+
+try {
+  await cmd("Page.navigate", { url: "http://localhost:5173/" });
+  await sleep(1500);
+  console.log("viewport:", await evaluate(`window.innerWidth + 'x' + window.innerHeight`));
+  await evaluate(`
+    window.__set = (el, v) => { Object.getOwnPropertyDescriptor(HTMLInputElement.prototype,'value').set.call(el, v); el.dispatchEvent(new Event('input',{bubbles:true})); };
+    window.__btn = (t, r=document) => [...r.querySelectorAll('button')].find(b => b.textContent.trim() === t);
+  `);
+  await evaluate(`(() => {
+    const i = [...document.querySelectorAll('input')];
+    window.__set(i.find(x => x.type === 'text' || x.type === 'email'), '[email protected]');
+    window.__set(document.querySelector('input[type=password]'), 'demo');
+    window.__btn('Sign in').click();
+  })()`);
+  await waitFor("document.querySelectorAll('.msg-row').length > 2", "the message list");
+  await sleep(1500);
+  await evaluate(`(() => { const r = document.querySelectorAll('.msg-row'); if (r[1]) r[1].click(); })()`);
+  await sleep(1500);
+
+  // The app's own control, the way a user switches theme.
+  await evaluate(`(() => {
+    const b = [...document.querySelectorAll('button')].find(x => /light mode/i.test(x.getAttribute('aria-label') || x.title || ''));
+    if (b) b.click(); else document.documentElement.dataset.theme = 'light';
+  })()`);
+  await sleep(2000);
+  const bg = await evaluate(`getComputedStyle(document.body).backgroundColor`);
+  const topbar = await evaluate(`getComputedStyle(document.querySelector('.topbar')).backgroundColor`);
+  console.log("body:", bg, "topbar:", topbar);
+  if (parseInt(bg.match(/\d+/)[0], 10) < 200) throw new Error("page is not rendering light");
+
+  const { data } = await cmd("Page.captureScreenshot", { format: "jpeg", quality: 82 });
+  await writeFile(`${OUT}/inbox-light.jpg`, Buffer.from(data, "base64"));
+  console.log("wrote inbox-light.jpg");
+} finally { ws.close(); chrome.kill(); }