fix(plugins): set is_core=True for all built-in plugins

All built-in plugins should be auto-activated. The is_core flag was
only set on some plugins, leaving Mail, DMS, Calendar, Automation,
Unified Search etc. inactive by default.

app/plugins/builtins/ai_proactive/plugin.py

@@ -42,7 +42,7 @@ class AIProactivePlugin(BasePlugin):
             "ai_proactive:write",
             "ai_proactive:config",
         ],
-        is_core=False,
+        is_core=True,
         settings_pages=[
             FrontendSettingsPage(path='ai-proactive', label_key='settings.aiProactive', label='Proactive AI', component='@/pages/ProactiveAISettings', icon='Sparkles', order=61),
         ],