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superseded, an in-flight ack for a superseded start, stop while pending,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>

web_interface/static/v3/app.js

@@ -356,9 +356,12 @@ window.previewPluginNow = function(pluginId) {
     })
     .then(r => r.json())
     .then(data => {
+        // 'starting' (202): the display service is starting and the request
+        // follows once it listens.
+        const starting = data.status === 'starting';
         showNotification(data.message || ('Previewing ' + pluginId + ' for 60 seconds'),
-                         data.status || 'success');
-        if (data.status === 'success') window.toggleFloatingPreview(true);
+                         starting ? 'info' : (data.status || 'success'));
+        if (data.status === 'success' || starting) window.toggleFloatingPreview(true);
     })
     .catch(err => {
         showNotification('Preview failed: ' + err.message, 'error');

web_interface/static/v3/plugins_manager.js

@@ -1739,6 +1739,13 @@ function submitOnDemandRequest(event) {
                 showNotification(`Requested on-demand mode for ${pluginName}`, 'success');
                 closeOnDemandModal();
                 setTimeout(() => loadOnDemandStatus(true), 700);
+            } else if (result.status === 'starting') {
+                // 202: the display service is starting; the web process sends
+                // the request once it listens. The status card shows the outcome.
+                const pluginName = resolvePluginDisplayName(currentOnDemandPluginId);
+                showNotification(`Starting the display for ${pluginName}…`, 'info');
+                closeOnDemandModal();
+                setTimeout(() => loadOnDemandStatus(true), 700);
             } else {
                 console.error('[submitOnDemandRequest] Request failed:', result);
                 showNotification(result.message || 'Failed to start on-demand mode', 'error');