fix: enable proper image files on drag-and-drop vs pasting base64 text

Replace HTML5 drag API with native file-based drag using tauri-plugin-drag.
Screenshots are now written to temp files and dragged as file:// URLs,
allowing proper image paste into Slack, browsers, and other apps.

Key changes:
- Update ScreenshotPreview to use pointer-based drag initiation
- Write temp file on popup mount, cleanup on unmount
- Cancel auto-dismiss timer when user interacts with popup
- Resize popup from 280x200 to 320x220 for better preview
- Disable drag-drop handler on popup window to prevent conflicts

Fixes #7

Author: codehakase

package-lock.json

@@ -23,6 +23,7 @@
     "tailwindcss": "^3.4.18"
   },
   "dependencies": {
+    "@crabnebula/tauri-plugin-drag": "^2.1.0",
     "@radix-ui/react-slot": "^1.2.3",
     "@tauri-apps/api": "^2.8.0",
     "@tauri-apps/plugin-global-shortcut": "^2.3.0",