feat(markdown): add paste detection with PasteChoicePopup for text/markdown/image

Simplify Ctrl+V shortcut to only handle recent internal copies, letting
native paste events flow to setupPaste for external clipboard content.
Extend setupPaste with text/HTML detection and popup callbacks. Wire
PasteChoicePopup in Editor.tsx with markdown (turndown HTML-to-MD),
plain text, and image paste choices.

frontend/src/canvas/image-drop.ts

@@ -324,11 +324,38 @@ export function setupPaste(
   onChange: OnChange,
   selection?: SelectionManager | null,
   uploads?: UploadManager | null,
+  opts?: {
+    onTextPaste?: (data: { text: string; html: string; hasImage: boolean }) => void;
+    onShortTextPaste?: (text: string) => void;
+  },
 ): () => void {
   async function onPaste(e: ClipboardEvent) {
     const items = e.clipboardData?.items;
     if (!items) return;
 
+    // ── Text/HTML detection — before media loop ──
+    let hasMedia = false;
+    let textContent = '';
+    let htmlContent = '';
+
+    for (let i = 0; i < items.length; i++) {
+      const item = items[i];
+      if (item.type.startsWith('image/') || item.type.startsWith('video/')) hasMedia = true;
+      if (item.type === 'text/plain') textContent = e.clipboardData?.getData('text/plain') || '';
+      if (item.type === 'text/html') htmlContent = e.clipboardData?.getData('text/html') || '';
+    }
+
+    if (textContent) {
+      e.preventDefault();
+      if (textContent.length > 20) {
+        opts?.onTextPaste?.({ text: textContent, html: htmlContent, hasImage: hasMedia });
+      } else {
+        opts?.onShortTextPaste?.(textContent);
+      }
+      return;
+    }
+
+    // ── Media paste logic (unchanged) ──
     const newItemIds: string[] = [];
 
     for (let i = 0; i < items.length; i++) {

frontend/src/canvas/shortcut-definitions.ts

@@ -493,36 +493,19 @@ export const shortcuts: ShortcutDef[] = [
     id: 'paste', keys: { key: 'v', ctrl: true },
     category: 'editing', description: 'Paste',
     handler: async (ctx) => {
-      // Strategy: Check system clipboard for images first.
-      // - If system clipboard has an image AND we did NOT just do an internal copy
-      //   (or it's been a while), paste from system clipboard (external image).
-      // - If we just did an internal copy (lastInternalCopyTime is recent),
-      //   use internal clipboard to duplicate scene items (preserves vector data).
-      // - If system clipboard has no images, fall back to internal clipboard.
-
+      // Only handle recent internal copies here.
+      // For external clipboard content (images, text, HTML), do NOT call
+      // e.preventDefault() — let the native paste event fire through to setupPaste
+      // in image-drop.ts, which is the single path for all external clipboard content.
       const timeSinceInternalCopy = Date.now() - _lastInternalCopyTime;
       const hasInternalItems = ctx.clipboardRef.current.length > 0;
       const recentInternalCopy = hasInternalItems && timeSinceInternalCopy < 500;
 
-      // If we JUST did an internal copy (<500ms ago), use internal clipboard
-      // (the system clipboard image is just the rasterized version of what we copied)
       if (recentInternalCopy) {
         await _pasteInternal(ctx);
         return;
       }
-
-      // Try system clipboard first
-      try {
-        const result = await ctx.pasteFromSystemClipboard();
-        if (result === 'Pasted image') return;
-      } catch {
-        // Clipboard API denied or unavailable — fall through
-      }
-
-      // Fall back to internal clipboard
-      if (hasInternalItems) {
-        await _pasteInternal(ctx);
-      }
+      // For external clipboard content: do nothing, let native paste event fire through to setupPaste
     },
   },
   {