feat(editor): paste and drop images into Markdown files (#489)

Pasting or dropping an image into a Markdown editor saves it beside the document in `assets/` and inserts a URL-encoded relative link as one undo step, in local and remote projects. Existing names get numbered suffixes; the folder and clipboard name pattern are in Settings › Editor › General. Non-Markdown editors, text paste and other dropped files keep their behavior.

Fixes #308

src/platform/editorSettings.ts

@@ -67,6 +67,8 @@ export const EDITOR_EXTRA_DEFAULTS = {
   boldFontWeight: 700,
   general: {
     wheelZoom: false,
+    markdownImageFolder: "assets",
+    markdownImageNamePattern: "image-{timestamp}",
     dragAndDrop: true,
     caretScroll: "minimize",
     softWrapFiles: "*.md; *.txt; *.rst; *.adoc",