feat(desktop): rich context menu — copy link, image, spellcheck

Replaces the hardcoded Copy/Paste/Select All menu with one driven by
ContextMenuParams: link items (Open in Browser, Copy Link Address),
image items (Copy Image, Save Image As), spellcheck suggestions, and
proper Cut/Copy/Paste in editable inputs. Both the main window and
the browser webview menus go through the shared helper.

MarkdownLink (web branch) now wraps its Pressable+Text in a real
<a href> with `display: contents` so Electron exposes linkURL on
right-click. Native branch is untouched.

packages/desktop/src/main.ts

@@ -23,6 +23,7 @@ import {
   setupWindowResizeEvents,
   setupDefaultContextMenu,
   setupDragDropPrevention,
+  buildStandardContextMenuItems,
 } from "./window/window-manager.js";
 import { registerDialogHandlers } from "./features/dialogs.js";
 import {
@@ -150,10 +151,7 @@ function showBrowserWebviewContextMenu(
   params: Electron.ContextMenuParams,
 ): void {
   const menu = Menu.buildFromTemplate([
-    { role: "copy", enabled: params.selectionText.length > 0 },
-    { role: "paste" },
-    { type: "separator" },
-    { role: "selectAll" },
+    ...buildStandardContextMenuItems(contents, params),
     ...(app.isPackaged
       ? []
       : [