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/ui/mod.rs

@@ -17,6 +17,7 @@ pub mod open_progress;
 pub mod overlay;
 pub mod page_setup;
 pub mod plotstyle;
+pub mod plugin_manager;
 pub mod properties;
 pub mod ribbon;
 pub mod scale_popup;

src/ui/plugin_manager.rs

@@ -0,0 +1,146 @@
+//! Plugin Manager window — lists the add-ons compiled into this build.
+//!
+//! Phase-1 stub: read-only inventory of installed plugins (name, version, id,
+//! API level, description). Enable/disable and dynamic loading come with the
+//! phase-2 loader; see `docs/plugin-architecture.md`.
+
+use crate::app::Message;
+use crate::plugin::manifest::PluginManifest;
+use iced::widget::{column, container, row, scrollable, text, Space};
+use iced::{Background, Border, Color, Element, Fill, Theme};
+
+// Register the command names for autocomplete.
+inventory::submit!(crate::command::CommandRegistration {
+    names: &["PLUGINS", "PLUGINMANAGER"]
+});
+
+const BG: Color = Color {
+    r: 0.15,
+    g: 0.15,
+    b: 0.15,
+    a: 1.0,
+};
+const CARD: Color = Color {
+    r: 0.12,
+    g: 0.12,
+    b: 0.12,
+    a: 1.0,
+};
+const BORDER: Color = Color {
+    r: 0.30,
+    g: 0.30,
+    b: 0.30,
+    a: 1.0,
+};
+const DIM: Color = Color {
+    r: 0.55,
+    g: 0.55,
+    b: 0.55,
+    a: 1.0,
+};
+const ACCENT: Color = Color {
+    r: 0.30,
+    g: 0.62,
+    b: 0.95,
+    a: 1.0,
+};
+const WHITE: Color = Color {
+    r: 0.92,
+    g: 0.92,
+    b: 0.92,
+    a: 1.0,
+};
+
+fn badge<'a>(label: String) -> Element<'a, Message> {
+    container(text(label).size(11).color(WHITE))
+        .padding([2, 8])
+        .style(|_: &Theme| container::Style {
+            background: Some(Background::Color(Color {
+                r: 0.20,
+                g: 0.34,
+                b: 0.52,
+                a: 1.0,
+            })),
+            border: Border {
+                radius: 4.0.into(),
+                ..Default::default()
+            },
+            ..Default::default()
+        })
+        .into()
+}
+
+fn plugin_card<'a>(m: &PluginManifest) -> Element<'a, Message> {
+    let header = row![
+        text(m.name.to_string()).size(15).color(WHITE),
+        Space::new().width(Fill),
+        badge(format!("v{}", m.version)),
+        Space::new().width(8),
+        badge(format!("API {}", m.api_version.major)),
+    ]
+    .align_y(iced::Center);
+
+    let id_line = text(m.id.to_string()).size(11).color(ACCENT);
+    let desc = text(m.description.to_string()).size(12).color(DIM);
+
+    let mut body = column![header, id_line, desc].spacing(5);
+
+    if !m.command_prefixes.is_empty() {
+        body = body.push(
+            text(format!("Commands: {}", m.command_prefixes.join(", ")))
+                .size(11)
+                .color(DIM),
+        );
+    }
+
+    container(body.padding([12, 14]))
+        .width(Fill)
+        .style(|_: &Theme| container::Style {
+            background: Some(Background::Color(CARD)),
+            border: Border {
+                color: BORDER,
+                width: 1.0,
+                radius: 6.0.into(),
+            },
+            ..Default::default()
+        })
+        .into()
+}
+
+pub fn view_window<'a>(plugins: &[&'static PluginManifest]) -> Element<'a, Message> {
+    let title = text("Installed Plugins").size(20).color(WHITE);
+    let subtitle = text(format!(
+        "{} add-on{} compiled into this build",
+        plugins.len(),
+        if plugins.len() == 1 { "" } else { "s" }
+    ))
+    .size(12)
+    .color(DIM);
+
+    let body: Element<'_, Message> = if plugins.is_empty() {
+        container(text("No plugins installed.").size(13).color(DIM))
+            .padding(20)
+            .into()
+    } else {
+        let mut list = column![].spacing(10);
+        for m in plugins {
+            list = list.push(plugin_card(m));
+        }
+        scrollable(list.width(Fill)).height(Fill).into()
+    };
+
+    container(
+        column![title, subtitle, Space::new().height(12), body]
+            .spacing(4)
+            .padding(20)
+            .width(Fill)
+            .height(Fill),
+    )
+    .style(|_: &Theme| container::Style {
+        background: Some(Background::Color(BG)),
+        ..Default::default()
+    })
+    .width(Fill)
+    .height(Fill)
+    .into()
+}