fix(generate): accept image drops from Explorer

[Yi-111-a]

Summary

• accept JPEG, PNG, and WebP drops on Generate image rows and persist the native path and preview
• expose Electron webUtils.getPathForFile through the preload bridge
• ignore stale image loads, surface unreadable files, and cover the preload bridge with a regression test

Testing

• npm run lint
• npm test (131 Python tests and 181 Node tests; 19 Python tests skipped)
• focused TypeScript check for the changed preload files
• built-in extension TypeScript build
• production Electron/Vite build

Manual Windows Explorer drag-and-drop verification was not available on this Linux runner.

Closes #344