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>

web_interface/blueprints/pages_v3.py

@@ -4,6 +4,7 @@ from html.parser import HTMLParser
 import json
 import logging
 import re
+import subprocess
 from pathlib import Path
 
 # Strict allowlists for URL-derived values used in path and script operations.
@@ -487,9 +488,25 @@ def _load_general_partial():
                              main_config=main_config,
                              auto_update_status=auto_update_status,
                              update_channel_status=update_channel_status,
+                             display_autostart=_display_autostart_state(),
                              web_login=_web_login_state())
 
 
+def _display_autostart_state():
+    """Whether ledmatrix.service starts at boot; None hides the toggle.
+
+    systemd's enablement is the setting itself (the installer enables the
+    unit), so nothing is stored in config.json. None when it cannot be read:
+    no systemctl (a dev machine) or the unit is not installed.
+    """
+    try:
+        result = subprocess.run(['systemctl', 'is-enabled', 'ledmatrix.service'],
+                                capture_output=True, text=True, timeout=5)
+    except (OSError, subprocess.SubprocessError):
+        return None
+    return {'enabled': True, 'disabled': False}.get(result.stdout.strip())
+
+
 def _web_login_state():
     """What the General tab's Security section shows; None hides it.
 

web_interface/static/v3/js/pages/general.js

@@ -191,6 +191,28 @@ export function revokeToken(ctx, id, name) {
     }).catch(failed(ctx));
 }
 
+/**
+ * The "Start Display on Boot" checkbox: enable or disable ledmatrix.service
+ * at once. A refusal (e.g. the sudo rule is missing) puts the box back.
+ */
+export function setDisplayAutostart(ctx, box) {
+    const on = box.checked;
+    box.disabled = true;
+    return send(ctx, 'POST', '/api/v3/system/action', { action: on ? 'enable_autostart' : 'disable_autostart' }).then(function(res) {
+        if (!res) return false;
+        const ok = res.ok && res.d.status === 'success';
+        if (!ok) box.checked = !on;
+        ctx.notify(ok ? (on ? 'The display will start on boot' : 'The display will not start on boot')
+                      : (res.d.message || 'Could not change display autostart'), ok ? 'success' : 'error');
+        return ok;
+    }).catch(function(err) {
+        box.checked = !on;
+        failed(ctx)(err);
+    }).finally(function() {
+        box.disabled = false;
+    });
+}
+
 const FORM_ACTIONS = { 'set-password': setPassword, 'disable-login': disableLogin, 'create-token': createToken };
 
 function drawTimezone(root, ctx) {
@@ -238,6 +260,10 @@ export function init(root, ctx) {
             if (row) revokeToken(ctx, row.dataset.tokenId, button.dataset.tokenName);
         }
     }, on);
+    root.addEventListener('change', function(event) {
+        const box = event.target;
+        if (box && box.dataset && box.dataset.action === 'display-autostart') setDisplayAutostart(ctx, box);
+    }, on);
     drawTimezone(root, ctx);
     active = ctx;
 }

web_interface/templates/v3/partials/general.html

@@ -12,6 +12,22 @@
 
     {{ ui.settings_filter() }}
 
+    {# Display autostart is systemd's enable/disable of ledmatrix.service, not a
+       config.json key: outside the form so Save never posts it. general.js
+       applies it on change. Hidden when the unit's state cannot be read. #}
+    {% if display_autostart is not none %}
+    <div class="form-group mb-6" id="setting-general-display_autostart" data-setting-key="display_autostart">
+        <label class="flex items-center">
+            <input type="checkbox"
+                   data-action="display-autostart"
+                   {% if display_autostart %}checked{% endif %}
+                   class="h-4 w-4 text-blue-600 focus:ring-blue-500 border-gray-300 rounded">
+            <span class="ml-2 text-sm font-medium text-gray-900">Start Display on Boot</span>
+            {{ ui.help_tip('Start the LED display automatically when the device powers on.\nApplies right away; no Save needed.\nWhen off, start it with Start Display on the Overview tab. The web interface still starts on boot.\nDefault: on.', 'Start Display on Boot') }}
+        </label>
+    </div>
+    {% endif %}
+
     <form hx-post="/api/v3/config/main"
           hx-ext="json-enc"
           hx-headers='{"Content-Type": "application/json"}'