feat(plugin): enable/disable plugins from the Plugin Manager

The manager was a read-only inventory. Each plugin now has an
Enable/Disable toggle: a disabled plugin keeps its manifest listed but
drops its ribbon tab and command dispatch. The disabled-id set persists
in settings.txt (disabled_plugins=) and the ribbon rebuilds on toggle.

Part of the #100 extensibility epic.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/app/mod.rs

@@ -293,6 +293,10 @@ pub(super) struct OpenCADStudio {
     modal_drag_last: Option<Point>,
     /// True while the modal title bar is held (a drag is in progress).
     modal_dragging: bool,
+    /// Plugin ids the user turned off in the Plugin Manager. Disabled plugins
+    /// keep their manifest listed but drop their ribbon tab and command
+    /// dispatch. Persisted via [`settings::UserSettings::disabled_plugins`].
+    disabled_plugins: rustc_hash::FxHashSet<String>,
     /// New-release notification window — opened on startup when the
     /// GitHub releases API reports a newer version than this build.
     /// First-launch "make Open CAD Studio the default for .dwg/.dxf?" prompt
@@ -1018,6 +1022,8 @@ pub enum Message {
     PluginManagerOpen,
     #[allow(dead_code)]
     PluginManagerClose,
+    /// Enable (`true`) or disable (`false`) the plugin with this id.
+    SetPluginEnabled(String, bool),
     // ── Quick Select / Select Similar ───────────────────────────────────
     /// Extend the current selection with every entity in the active
     /// layout that matches a selected entity by (type, layer).
@@ -1428,6 +1434,7 @@ impl OpenCADStudio {
             modal_offset: iced::Vector::ZERO,
             modal_drag_last: None,
             modal_dragging: false,
+            disabled_plugins: rustc_hash::FxHashSet::default(),
             default_assoc_prompted: false,
             update_notice_version: None,
             update_notice_body: None,

src/app/settings.rs

@@ -74,6 +74,10 @@ pub struct UserSettings {
     /// prompt has already been shown. Set once the user answers (either way),
     /// so we never nag again on subsequent launches.
     pub default_assoc_prompted: bool,
+    /// Ids of plugins the user turned off in the Plugin Manager. Disabled
+    /// plugins keep their manifest listed but drop their ribbon tab and command
+    /// dispatch.
+    pub disabled_plugins: Vec<String>,
 }
 
 impl Default for UserSettings {
@@ -96,6 +100,7 @@ impl Default for UserSettings {
                 SnapType::Nearest,
             ],
             default_assoc_prompted: false,
+            disabled_plugins: Vec::new(),
         }
     }
 }
@@ -130,6 +135,14 @@ impl UserSettings {
                 "osnap" => s.snap_enabled = val == "1",
                 "otrack" => s.otrack = val == "1",
                 "default_assoc_prompted" => s.default_assoc_prompted = val == "1",
+                "disabled_plugins" => {
+                    s.disabled_plugins = val
+                        .split(',')
+                        .map(|t| t.trim())
+                        .filter(|t| !t.is_empty())
+                        .map(|t| t.to_string())
+                        .collect();
+                }
                 "snap_modes" => {
                     let modes: Vec<SnapType> =
                         val.split(',').filter_map(|t| snap_from_id(t.trim())).collect();
@@ -155,7 +168,7 @@ impl UserSettings {
             .collect::<Vec<_>>()
             .join(",");
         let body = format!(
-            "dyn={}\northo={}\npolar={}\npolar_increment_deg={}\ngrid={}\nosnap={}\notrack={}\ndefault_assoc_prompted={}\nsnap_modes={}\n",
+            "dyn={}\northo={}\npolar={}\npolar_increment_deg={}\ngrid={}\nosnap={}\notrack={}\ndefault_assoc_prompted={}\nsnap_modes={}\ndisabled_plugins={}\n",
             b(self.dyn_input),
             b(self.ortho),
             b(self.polar),
@@ -165,6 +178,7 @@ impl UserSettings {
             b(self.otrack),
             b(self.default_assoc_prompted),
             modes,
+            self.disabled_plugins.join(","),
         );
         let _ = std::fs::write(path, body);
     }

src/app/update.rs

@@ -141,6 +141,11 @@ impl OpenCADStudio {
             otrack: self.snapper.otrack_enabled,
             snap_modes: super::settings::UserSettings::modes_from(self.snapper.enabled.iter()),
             default_assoc_prompted: self.default_assoc_prompted,
+            disabled_plugins: {
+                let mut v: Vec<String> = self.disabled_plugins.iter().cloned().collect();
+                v.sort();
+                v
+            },
         }
     }
 
@@ -155,6 +160,22 @@ impl OpenCADStudio {
         self.snapper.otrack_enabled = s.otrack;
         self.snapper.enabled = s.snap_modes.iter().copied().collect();
         self.default_assoc_prompted = s.default_assoc_prompted;
+        self.disabled_plugins = s.disabled_plugins.iter().cloned().collect();
+        self.rebuild_ribbon_modules();
+    }
+
+    /// Rebuild the ribbon's tab list from the registry, dropping the tabs of any
+    /// disabled plugins. Call after `disabled_plugins` changes.
+    pub(super) fn rebuild_ribbon_modules(&mut self) {
+        let modules =
+            crate::plugin::ribbon_modules_enabled(&self.disabled_plugins);
+        self.ribbon.set_modules(modules);
+    }
+
+    /// Snapshot of disabled plugin ids — lets the registry skip them while it
+    /// holds a `&mut` borrow of the app via `HostSession`.
+    pub(crate) fn disabled_plugin_ids(&self) -> rustc_hash::FxHashSet<String> {
+        self.disabled_plugins.clone()
     }
 
     /// Write preferences to disk only when they differ from the last write,
@@ -5432,6 +5453,16 @@ impl OpenCADStudio {
                 self.close_active_modal();
                 Task::none()
             }
+            Message::SetPluginEnabled(id, enabled) => {
+                if enabled {
+                    self.disabled_plugins.remove(&id);
+                } else {
+                    self.disabled_plugins.insert(id);
+                }
+                self.rebuild_ribbon_modules();
+                self.persist_settings_if_changed();
+                Task::none()
+            }
 
             Message::EnterViewport(handle) => {
                 let i = self.active_tab;

src/app/view.rs

@@ -1120,7 +1120,10 @@ impl OpenCADStudio {
                 sized(crate::ui::shortcuts::view_window(&self.shortcut_overrides), 720, 520)
             }
             super::ModalKind::PluginManager => sized(
-                crate::ui::plugin_manager::view_window(&crate::plugin::installed_manifests()),
+                crate::ui::plugin_manager::view_window(
+                    &crate::plugin::installed_manifests(),
+                    &self.disabled_plugins,
+                ),
                 520,
                 460,
             ),