refboard: restore clipboard copy and native paste flows

frontend/src/canvas/clipboard.ts

@@ -61,8 +61,7 @@ export async function writeCanvasToClipboard(
     outputCanvas.width = sw;
     outputCanvas.height = sh;
     const ctx2d = outputCanvas.getContext('2d')!;
-    ctx2d.fillStyle = '#1e1e1e';
-    ctx2d.fillRect(0, 0, outputCanvas.width, outputCanvas.height);
+    ctx2d.clearRect(0, 0, outputCanvas.width, outputCanvas.height);
     ctx2d.drawImage(sourceCanvas, sx, sy, sw, sh, 0, 0, sw, sh);
   } else {
     // Full current view copy.
@@ -70,8 +69,7 @@ export async function writeCanvasToClipboard(
     outputCanvas.width = sourceCanvas.width;
     outputCanvas.height = sourceCanvas.height;
     const ctx2d = outputCanvas.getContext('2d')!;
-    ctx2d.fillStyle = '#1e1e1e';
-    ctx2d.fillRect(0, 0, outputCanvas.width, outputCanvas.height);
+    ctx2d.clearRect(0, 0, outputCanvas.width, outputCanvas.height);
     ctx2d.drawImage(sourceCanvas, 0, 0);
   }
 

frontend/src/canvas/shortcut-definitions.ts

@@ -492,6 +492,7 @@ export const shortcuts: ShortcutDef[] = [
   {
     id: 'paste', keys: { key: 'v', ctrl: true },
     category: 'editing', description: 'Paste',
+    preventDefault: false,
     handler: async (ctx) => {
       // Only handle recent internal copies here.
       // For external clipboard content (images, text, HTML), do NOT call

frontend/src/canvas/shortcuts.ts

@@ -24,6 +24,7 @@ export interface ShortcutDef {
   category: 'alignment' | 'arrangement' | 'normalize' | 'navigation' | 'editing' | 'view' | 'tools' | 'image';
   needsSelection?: boolean;   // skip if no objects selected
   minSelection?: number;      // minimum selected objects (default 1 if needsSelection)
+  preventDefault?: boolean;   // defaults to true
 }
 
 export interface ShortcutContext {

frontend/src/hooks/useShortcutHandler.ts

@@ -141,7 +141,9 @@ export function useShortcutHandler(deps: ShortcutHandlerDeps) {
         if (def.needsSelection && activeCount === 0) continue;
         if (def.minSelection && activeCount < def.minSelection) continue;
 
+        if (def.preventDefault !== false) {
           e.preventDefault();
+        }
         await def.handler(ctx);
 
         // Update zoom display after any action