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/test_general_display_autostart.py

@@ -0,0 +1,60 @@
+"""The General tab's "Start Display on Boot" toggle reads systemd's state.
+
+ledmatrix.service enabled -> checked, disabled -> unchecked, unreadable (no
+systemctl, unit not installed) -> no toggle. The toggle itself posts the
+existing enable_autostart / disable_autostart system actions, whose sudo
+grants test_systemctl_sudoers_alignment.py already checks.
+"""
+import subprocess
+import sys
+from pathlib import Path
+
+import pytest
+
+PROJECT_ROOT = Path(__file__).parent.parent
+sys.path.insert(0, str(PROJECT_ROOT))
+
+from test.test_web_settings_ui import client  # noqa: F401,E402
+from web_interface.blueprints import pages_v3  # noqa: E402
+
+
+REAL_RUN = subprocess.run
+UNIT_QUERY = ['systemctl', 'is-enabled', 'ledmatrix.service']
+
+
+def _fake_systemctl(monkeypatch, answer):
+    """Answer the unit query with `answer` (stdout, or an exception to
+    raise); every other command (the update channel's git) runs for real."""
+    calls = []
+
+    def run(cmd, *args, **kwargs):
+        if cmd != UNIT_QUERY:
+            return REAL_RUN(cmd, *args, **kwargs)
+        calls.append(cmd)
+        if isinstance(answer, Exception):
+            raise answer
+        return subprocess.CompletedProcess(cmd, 0, stdout=answer, stderr='')
+    monkeypatch.setattr(pages_v3.subprocess, 'run', run)
+    return calls
+
+
+def _toggle(body):
+    start = body.find('data-action="display-autostart"')
+    return None if start < 0 else body[start:body.find('>', start)]
+
+
+@pytest.mark.parametrize('stdout, checked', [('enabled\n', True), ('disabled\n', False)])
+def test_toggle_reflects_the_unit(client, monkeypatch, stdout, checked):
+    calls = _fake_systemctl(monkeypatch, stdout)
+    tag = _toggle(client.get('/v3/partials/general').get_data(as_text=True))
+    assert calls == [UNIT_QUERY]
+    assert tag is not None
+    assert ('checked' in tag) is checked
+
+
+@pytest.mark.parametrize('outcome', [FileNotFoundError(), 'not-found\n'])
+def test_toggle_hidden_when_state_unreadable(client, monkeypatch, outcome):
+    _fake_systemctl(monkeypatch, outcome)
+    body = client.get('/v3/partials/general').get_data(as_text=True)
+    assert _toggle(body) is None
+    assert 'name="plugins_directory"' in body  # rest of the tab still renders