feat(web): Start Display on Boot toggle on the General tab

Turns ledmatrix.service's autostart on or off at once through the existing
enable_autostart / disable_autostart system actions (sudo grants already in
lib_sudoers.sh). The checkbox shows systemd's own state, read with
`systemctl is-enabled`, so nothing is stored in config.json and the default
stays on. Hidden when the state cannot be read; a refused change puts the
box back and shows the route's message.

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

test/js/unit/test_general_display_autostart.js

@@ -0,0 +1,77 @@
+// The General tab's "Start Display on Boot" checkbox (setDisplayAutostart in
+// static/v3/js/pages/general.js) posts enable_autostart / disable_autostart
+// to /api/v3/system/action and puts the box back when that is refused --
+// including the route's HTTP-200 {status: "error"} answer for a failed
+// systemctl (e.g. the sudo rule is missing).
+//
+// Fake fetch, no jsdom and no server needed.
+
+const path = require('path');
+const { pathToFileURL } = require('url');
+
+const JS = path.resolve(__dirname, '../../../web_interface/static/v3/js');
+const load = f => import(pathToFileURL(path.join(JS, f)).href);
+
+let pass = 0, fail = 0;
+const ok = (label, cond, extra) => cond
+  ? (pass++, console.log('  ok   ' + label))
+  : (fail++, console.log('  FAIL ' + label + (extra !== undefined ? '  ' + JSON.stringify(extra) : '')));
+
+async function run(checked, answer) {
+  const { createApi } = await load('core/api.js');
+  const general = await load('pages/general.js');
+  const sent = [];
+  const notes = [];
+  const fetch = (url, init) => {
+    sent.push([url, JSON.parse(init.body)]);
+    if (answer === 'network') return Promise.reject(new TypeError('Failed to fetch'));
+    return Promise.resolve({
+      ok: answer.ok, status: answer.ok ? 200 : 500,
+      headers: { get: () => null },
+      text: () => Promise.resolve(JSON.stringify(answer.body)),
+    });
+  };
+  const ctx = { root: {}, state: {}, signal: { aborted: false },
+                api: createApi({ fetch }), notify: (m, t) => notes.push([m, t]) };
+  const box = { checked, disabled: false };
+  await general.setDisplayAutostart(ctx, box);
+  return { box, sent, notes };
+}
+
+(async () => {
+  console.log('\n── General tab: Start Display on Boot ──');
+
+  {
+    const t = await run(false, { ok: true, body: { status: 'success', message: 'ok' } });
+    ok('unchecking posts disable_autostart',
+       t.sent.length === 1 && t.sent[0][0] === '/api/v3/system/action' && t.sent[0][1].action === 'disable_autostart', t.sent);
+    ok('...stays unchecked and re-enabled', t.box.checked === false && t.box.disabled === false);
+    ok('...and reports success', t.notes.some(([, type]) => type === 'success'), t.notes);
+  }
+
+  {
+    const t = await run(true, { ok: true, body: { status: 'success' } });
+    ok('checking posts enable_autostart', t.sent[0][1].action === 'enable_autostart', t.sent);
+  }
+
+  {
+    const t = await run(false, { ok: true, body: { status: 'error', message: 'sudo: a password is required' } });
+    ok('a failed systemctl (HTTP 200, status error) puts the box back', t.box.checked === true && !t.box.disabled);
+    ok('...and shows the route\'s message',
+       t.notes.some(([m, type]) => type === 'error' && /sudo/.test(m)), t.notes);
+  }
+
+  {
+    const t = await run(true, { ok: false, body: { status: 'error', message: 'boom' } });
+    ok('an HTTP error puts the box back', t.box.checked === false && !t.box.disabled);
+  }
+
+  {
+    const t = await run(true, 'network');
+    ok('a network failure puts the box back', t.box.checked === false && !t.box.disabled);
+    ok('...and reports it', t.notes.some(([, type]) => type === 'error'), t.notes);
+  }
+
+  console.log(`\n${pass} passed, ${fail} failed\n`);
+  process.exit(fail ? 1 : 0);
+})().catch(e => { console.log('HARNESS ERROR: ' + e.stack); process.exit(1); });