feat(web): a start that waits for the display answers 202 and is delivered in the background

The start route held a request open for up to 45 s while a cold-started
display loaded its plugins; the MQTT bridge (15 s timeout) and browsers
reported a failure for a request that was then delivered.

Now, when no display is listening, the route starts the service if asked
and answers 202 with status "starting" at once. A single worker in the web
process (web_interface/on_demand_dispatch.py) sends the request until the
display acknowledges it or the wait runs out (45 s cold start, 10 s for a
running service without a socket yet). A newer start supersedes the
pending one; a stop cancels it (and succeeds, with cancelled_request_id,
even with no display listening). The outcome is reported by
/display/on-demand/status (source "web": starting, or error with
start-timeout or the socket's reason, until the display publishes
something newer) and by /display/current-status as on_demand_pending.

Callers: the web UI's on-demand modal and "Preview on display" treat
"starting" as taken (an info toast); the MQTT bridge already treats any
non-error 2xx as success (now pinned by a test).

Tests: the dispatcher (ack, retry then ack, start-timeout, other failures,
superseded, an in-flight ack for a superseded start, stop while pending,
a per-start wait, outcome lifetime); the routes (202, status routes while
pending and after a timeout, a later display state replacing the failure,
stop while pending, a new start superseding); a JS suite for app.js.
Mutation check: 20 mutants on the worker, the routes and app.js, 20 killed.

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

test/js/unit/test_on_demand_starting.js

@@ -0,0 +1,69 @@
+// POST /api/v3/display/on-demand/start answers 202 with status "starting"
+// when the display service has to be started first: the request is taken,
+// and the web process sends it once the display listens. "Preview on
+// display" (app.js) must read that as taken -- an info toast and the
+// floating preview opened -- not as a failure. Runs the shipped app.js in a
+// vm with a minimal fake DOM, as test_restart_banner.js does.
+
+const fs = require('fs');
+const path = require('path');
+const vm = require('vm');
+const V3 = path.resolve(__dirname, '../../../web_interface/static/v3');
+
+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) : '')));
+
+function load(answer) {
+  const notes = [];
+  const opened = [];
+  const noop = () => {};
+  const document = {
+    body: { addEventListener: noop },
+    addEventListener: noop,
+    getElementById: () => null,
+    querySelector: () => null,
+    querySelectorAll: () => [],
+  };
+  const window = { addEventListener: noop, getApp: () => null };
+  const context = {
+    window, document, console,
+    sessionStorage: { setItem: noop, removeItem: noop, getItem: () => null },
+    showNotification: (m, t) => notes.push([m, t]),
+    setTimeout: () => 0,
+    fetch: () => Promise.resolve({ json: () => Promise.resolve(answer) }),
+  };
+  vm.createContext(context);
+  vm.runInContext(fs.readFileSync(path.join(V3, 'app.js'), 'utf8'), context);
+  window.toggleFloatingPreview = (open) => opened.push(open);
+  return { window, notes, opened };
+}
+
+async function preview(answer) {
+  const t = load(answer);
+  t.window.previewPluginNow('weather');
+  for (let i = 0; i < 5; i++) await Promise.resolve();
+  return t;
+}
+
+(async () => {
+  console.log('\npreviewPluginNow');
+  {
+    const t = await preview({ status: 'starting', message: 'The display service is starting',
+                              data: { request_id: 'r1', pending: true } });
+    ok('a 202 "starting" answer is an info toast, not an error',
+       t.notes.length === 1 && t.notes[0][1] === 'info', t.notes);
+    ok('and the preview opens', t.opened.length === 1 && t.opened[0] === true, t.opened);
+  }
+  {
+    const t = await preview({ status: 'success', data: { request_id: 'r1' } });
+    ok('a 200 success still opens it', t.opened.length === 1 && t.notes[0][1] === 'success', t.notes);
+  }
+  {
+    const t = await preview({ status: 'error', message: 'no display' });
+    ok('an error does not', t.opened.length === 0 && t.notes[0][1] === 'error', t.notes);
+  }
+  console.log(`\n${pass} passed, ${fail} failed`);
+  process.exit(fail ? 1 : 0);
+})();