feat: plugin install/uninstall lifecycle + UI buttons

- AppConfig: add InstalledPlugins []string
- Manager.Discover(): no config dependency, all plugins start inactive
- Manager.SyncConfig(): apply installed/enabled state from AppConfig
- Manager.Enable(): works for plugins without on_install hook
- Manager.Install/Uninstall(): run on_install/on_uninstall hooks
- ActivatePlugin: skip if HasInstall && !Installed
- ReloadPlugins: Discover → SyncConfig → InitRuntimes
- Bindings: InstallPlugin, UninstallPlugin
- SettingsPlugins: install/uninstall buttons, toggle only after install
- Calendar: migration moved from on_init to on_install, on_uninstall drops tables
- Tests: all 12 pass (manager + runtime + calendar)

internal/core/config/appconfig.go

@@ -21,6 +21,7 @@ type AppConfig struct {
 	Language          string         `json:"language"`
 	EnabledTemplates  []string       `json:"enabled_templates"`
 	EnabledPlugins    []string       `json:"enabled_plugins"`
+	InstalledPlugins  []string       `json:"installed_plugins"`
 	FirstRunCompleted bool           `json:"first_run_completed"`
 	Window            WindowConfig   `json:"window,omitempty"`
 	Vault             VaultAppConfig `json:"vault,omitempty"`