fix: resolve 15 critical/high/medium issues from self-review

CRITICAL fixes:
- #14: Render source image on canvas via Konva Image + use-image hook
- #3: Wire move tool handlers (onClick, onDragEnd, onTransformEnd, draggable)
  to all CanvasObjectRenderer shapes
- #4: Implement image object rendering for fill/gradient output
- #2: Show fallback text in histogram panel when no imageData provided
- #1: Forward all args in zundo handleSet debounce wrapper

HIGH fixes:
- #5: Track raw screen cursor position for brush overlay instead of
  using canvas-space coordinates
- #6: Export dialog uses Konva stage.toDataURL via module-level ref
  instead of DOM querySelector for correct export at any zoom/pan
- #7: Add _historyVersion increment to setAdjustment, resetAdjustments,
  toggleFilter, and setFilterParam for undo tracking
- #8: Include lastAction in partialize so history labels display correctly

MEDIUM fixes:
- #10: Move useEditorShortcuts from EditorCanvas to EditorPage with
  save/export callbacks
- #11: Remove _historyVersion increment from updateObject to prevent
  brush strokes from flooding undo history
- #12: Apply Konva filters (Brighten, Contrast, HSL, Blur, Grayscale,
  Sepia, Invert, Pixelate, Emboss, Posterize, Noise, Solarize,
  Threshold, Kaleidoscope) to source image node based on store state

apps/web/src/components/editor/panels/histogram-panel.tsx

@@ -116,6 +116,7 @@ interface HistogramPanelProps {
   imageData?: ImageData | null;
 }
 
+// Issue #2: When no imageData is provided, show a placeholder message
 export function HistogramPanel({ imageData }: HistogramPanelProps) {
   const canvasRef = useRef<HTMLCanvasElement>(null);
   const [stats, setStats] = useState<HistogramStats | null>(null);
@@ -176,6 +177,11 @@ export function HistogramPanel({ imageData }: HistogramPanelProps) {
         className="w-full rounded border border-border"
         style={{ imageRendering: "pixelated" }}
       />
+      {!imageData && !stats && (
+        <p className="text-[10px] text-muted-foreground text-center py-1">
+          No histogram data available
+        </p>
+      )}
       {stats && (
         <div className="grid grid-cols-3 gap-1 text-[10px] text-muted-foreground">
           <div>