fix: use correct error message when user-installed plugin is outdated

[datlechin]

Summary

• When a user-installed plugin has a lower PluginKit version than the app requires, the error message was misleading — it said "Plugin requires PluginKit version 3, but app provides version 2" when the reality was the opposite (app requires v3, plugin was built with v2)
• Added a dedicated pluginOutdated error case with a clear message: "Plugin was built with PluginKit version X, but version Y is required. Please update the plugin."
• Updated 3 call sites in PluginManager (2 throws + 1 log message) to use the new error case

Test plan

• Install a plugin built with an older PluginKit version and verify the new error message appears correctly
• Verify plugins with matching PluginKit versions still load normally
• Verify plugins with a newer PluginKit version than the app still show the original incompatibleVersion message