feat: add image drag and drop to Kanban card description

Add ability to drag and drop images into the description section when creating
new Kanban cards. Images are saved to a temp directory and their paths are stored
with the feature for agent context.

- Create DescriptionImageDropZone component with drag/drop support
- Integrate with Add Feature dialog in board-view
- Add FeatureImagePath interface to track temp file paths
- Update saveFeatures to persist imagePaths
- Add saveImageToTemp to mock electron API
- Add test utilities for image upload testing

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/src/store/app-store.ts

@@ -44,6 +44,13 @@ export interface FeatureImage {
   size: number;
 }
 
+export interface FeatureImagePath {
+  id: string;
+  path: string; // Path to the temp file
+  filename: string;
+  mimeType: string;
+}
+
 export interface Feature {
   id: string;
   category: string;
@@ -51,6 +58,7 @@ export interface Feature {
   steps: string[];
   status: "backlog" | "in_progress" | "verified";
   images?: FeatureImage[];
+  imagePaths?: FeatureImagePath[]; // Paths to temp files for agent context
   startedAt?: string; // ISO timestamp for when the card moved to in_progress
   skipTests?: boolean; // When true, skip TDD approach and require manual verification
 }