feat(plugin): add a Plugin Manager window listing installed add-ons

Read-only inventory of the add-ons compiled into the build — name, version,
id, API level, description, command prefixes — built from the extracted
`ocs_plugin_api` manifest. Opened with the `PLUGINS` / `PLUGINMANAGER` command
or the Start-page "Plugins" button.

Adds `installed_manifests()` to the plugin registry and wires the window the
same way as the existing About / Shortcuts windows.

Part of #100 (phase-1 plugin manager UI stub).

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

src/app/commands.rs

@@ -36,6 +36,7 @@ impl OpenCADStudio {
             && !matches!(
                 cmd,
                 "NEW" | "OPEN" | "EXIT" | "QUIT" | "REPORT" | "CHANGELOG" | "ABOUT"
+                    | "PLUGINS" | "PLUGINMANAGER"
             )
         {
             self.command_line
@@ -2675,6 +2676,10 @@ impl OpenCADStudio {
                 return Task::done(Message::AboutOpen);
             }
 
+            "PLUGINS" | "PLUGINMANAGER" => {
+                return Task::done(Message::PluginManagerOpen);
+            }
+
             "CHANGELOG" => {
                 let _ = open::that("https://github.com/HakanSeven12/OpenCADStudio/releases");
                 self.command_line.push_info("Opening release notes...");

src/app/mod.rs

@@ -292,6 +292,7 @@ pub(super) struct OpenCADStudio {
     color_pick_target: Option<ColorPickTarget>,
     shortcuts_window: Option<window::Id>,
     about_window: Option<window::Id>,
+    plugin_manager_window: Option<window::Id>,
     /// New-release notification window — opened on startup when the
     /// GitHub releases API reports a newer version than this build.
     update_notice_window: Option<window::Id>,
@@ -987,6 +988,10 @@ pub enum Message {
     // ── About window ────────────────────────────────────────────────────
     AboutOpen,
     AboutCopyInfo,
+    // ── Plugin Manager window ───────────────────────────────────────────
+    PluginManagerOpen,
+    #[allow(dead_code)]
+    PluginManagerClose,
     // ── Quick Select / Select Similar ───────────────────────────────────
     /// Extend the current selection with every entity in the active
     /// layout that matches a selected entity by (type, layer).
@@ -1402,6 +1407,7 @@ impl OpenCADStudio {
             color_pick_target: None,
             shortcuts_window: None,
             about_window: None,
+            plugin_manager_window: None,
             update_notice_window: None,
             assoc_prompt_window: None,
             default_assoc_prompted: false,
@@ -1712,6 +1718,9 @@ pub fn run() -> iced::Result {
         if Some(window_id) == state.about_window {
             return "About Open CAD Studio".into();
         }
+        if Some(window_id) == state.plugin_manager_window {
+            return "Plugin Manager".into();
+        }
         if Some(window_id) == state.update_notice_window {
             return "Update Available".into();
         }

src/app/update.rs

@@ -1640,6 +1640,9 @@ impl OpenCADStudio {
                     self.about_window = None;
                     self.ribbon.deactivate_tool_if("ABOUT");
                 }
+                if self.plugin_manager_window == Some(id) {
+                    self.plugin_manager_window = None;
+                }
                 if self.update_notice_window == Some(id) {
                     self.update_notice_window = None;
                 }
@@ -5394,6 +5397,28 @@ impl OpenCADStudio {
                 iced::clipboard::write(info)
             }
 
+            // ── Plugin Manager window ─────────────────────────────────────
+            Message::PluginManagerOpen => {
+                if let Some(id) = self.plugin_manager_window {
+                    return window::gain_focus(id);
+                }
+                let (id, task) = window::open(window::Settings {
+                    size: iced::Size::new(520.0, 460.0),
+                    resizable: true,
+                    level: window::Level::AlwaysOnTop,
+                    ..Default::default()
+                });
+                self.plugin_manager_window = Some(id);
+                task.map(|_| Message::Noop)
+            }
+            Message::PluginManagerClose => {
+                if let Some(id) = self.plugin_manager_window.take() {
+                    window::close(id)
+                } else {
+                    Task::none()
+                }
+            }
+
             Message::EnterViewport(handle) => {
                 let i = self.active_tab;
                 // Clear paper-space selection before entering model space.

src/app/view.rs

@@ -474,6 +474,9 @@ impl OpenCADStudio {
         if Some(window_id) == self.about_window {
             return crate::ui::about::view_window();
         }
+        if Some(window_id) == self.plugin_manager_window {
+            return crate::ui::plugin_manager::view_window(&crate::plugin::installed_manifests());
+        }
         if Some(window_id) == self.update_notice_window {
             let latest = self.update_notice_version.as_deref().unwrap_or("?");
             let body = self.update_notice_body.as_deref().unwrap_or("");
@@ -3743,6 +3746,7 @@ pub(super) fn start_page_view<'a>() -> Element<'a, Message> {
                 event: crate::modules::ModuleEvent::Command("CHANGELOG".to_string()),
             },
         ),
+        outline_btn("Plugins", Message::PluginManagerOpen),
         outline_btn("About", Message::AboutOpen),
     ]
     .spacing(12)