fix(web): remove the General-tab plugin system toggles that did nothing

plugin_system.auto_discover, auto_load_enabled and development_mode had
General-tab toggles whose help tips promised dormant plugins and verbose
logging, but nothing reads them: every enabled plugin is discovered and
loaded regardless. Remove the three toggles.

The keys stay tolerated in stored configs. The save handler now stores
a flag only when a client sends it; treating a missing key as an
unchecked box would otherwise rewrite all three to false on every
General-tab save, which still posts plugins_directory.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

test/test_plugin_system_legacy_flags.py

@@ -0,0 +1,70 @@
+"""plugin_system.auto_discover / auto_load_enabled / development_mode.
+
+The General tab offered three toggles for these, with help tips promising
+"plugins installed but dormant" and "verbose logging". Nothing in src/,
+web_interface/ or scripts/ reads them: every enabled plugin is discovered and
+loaded regardless. The toggles are gone; the keys stay tolerated in stored
+configs, and saving the General tab must not rewrite them.
+"""
+import copy
+import json
+import sys
+from pathlib import Path
+
+import pytest
+
+PROJECT_ROOT = Path(__file__).parent.parent
+sys.path.insert(0, str(PROJECT_ROOT))
+
+from test._api_v3_test_helpers import api_v3_client, api_v3_module  # noqa: F401,E402
+from test.test_web_settings_ui import REALISTIC_CONFIG, client  # noqa: F401,E402
+
+LEGACY_FLAGS = ('auto_discover', 'auto_load_enabled', 'development_mode')
+
+
+def test_the_general_tab_no_longer_offers_the_toggles(client):
+    body = client.get('/v3/partials/general').get_data(as_text=True)
+    for flag in LEGACY_FLAGS:
+        assert f'name="{flag}"' not in body, flag
+        assert f'setting-general-{flag}' not in body, flag
+    # The setting that does work stays.
+    assert 'name="plugins_directory"' in body
+
+
+@pytest.fixture
+def saved(api_v3_module, monkeypatch):
+    captured = {}
+    stored = copy.deepcopy(REALISTIC_CONFIG)
+    stored['plugin_system'].update(
+        auto_discover=True, auto_load_enabled=True, development_mode=True)
+    api_v3_module.api_v3.config_manager.load_config.return_value = stored
+
+    def fake_save(_manager, config, **_kwargs):
+        captured['config'] = config
+        return True, ''
+
+    monkeypatch.setattr(api_v3_module, '_save_config_atomic', fake_save)
+    return captured
+
+
+def test_saving_the_general_form_leaves_stored_flags_alone(api_v3_client, saved):
+    # What the General form posts now: no checkbox fields for the flags.
+    resp = api_v3_client.post('/api/v3/config/main', data={
+        'timezone': 'America/Chicago', 'city': 'Dallas', 'state': 'Texas',
+        'country': 'US', 'plugins_directory': 'plugin-repos',
+    })
+    assert resp.status_code == 200, resp.get_json()
+    plugin_system = saved['config']['plugin_system']
+    # Missing used to mean "unchecked" and saved all three as false.
+    assert all(plugin_system[flag] is True for flag in LEGACY_FLAGS), plugin_system
+    assert plugin_system['plugins_directory'] == 'plugin-repos'
+
+
+def test_an_api_client_can_still_store_a_flag(api_v3_client, saved):
+    resp = api_v3_client.post('/api/v3/config/main', data=json.dumps({
+        'timezone': 'America/Chicago', 'development_mode': False,
+    }), content_type='application/json')
+    assert resp.status_code == 200, resp.get_json()
+    plugin_system = saved['config']['plugin_system']
+    assert plugin_system['development_mode'] is False
+    assert plugin_system['auto_discover'] is True