Add plugin system with sample plugin and settings UI

Introduces a plugin architecture, including a PluginManager, plugin loading in main and renderer processes, and a sample plugin demonstrating menu, IPC, and context menu contributions. Adds a Plugins tab to the settings UI for managing plugins (enable/disable, reload), and updates preload.js to load renderer preloads from plugins. Documentation for plugin development is included in README-PLUGINS.md.

preload.js

@@ -130,4 +130,27 @@ contextBridge.exposeInMainWorld('downloadsAPI', {
   onUpdated: (handler) => ipcRenderer.on('downloads-updated', (_e, payload) => handler(payload)),
   onDone: (handler) => ipcRenderer.on('downloads-done', (_e, payload) => handler(payload)),
   onCleared: (handler) => ipcRenderer.on('downloads-cleared', handler)
-});
+});
+
+// ----------------------------------------
+// Plugin renderer preloads
+// ----------------------------------------
+// We request a list of absolute file paths from main and require() them here.
+// Each file can optionally call contextBridge.exposeInMainWorld to add APIs.
+(async () => {
+  try {
+    const preloads = await ipcRenderer.invoke('plugins-get-renderer-preloads');
+    if (Array.isArray(preloads)) {
+      for (const p of preloads) {
+        try {
+          // eslint-disable-next-line global-require, import/no-dynamic-require
+          require(p);
+        } catch (e) {
+          console.error('[Plugins] Failed to load renderer preload:', p, e);
+        }
+      }
+    }
+  } catch (e) {
+    console.warn('[Plugins] No renderer preloads:', e);
+  }
+})();