feat(desktop): drag workspace files into composer

[Blue-Berrys]

Problem

Files shown in the bundled File Manager could not be dragged into the Composer, so users had to type a path or use the file picker instead.

Result

Regular, non-symlink file rows can now be dragged from the work panel into the Composer. A drop inserts one removable file chip using the workspace-relative path and leaves the source file in place. Direct and cross-WebContents delivery are deduplicated so one gesture creates one chip; directories remain navigation-only.

Closes #745

Validation

• Manually verified on macOS with the bundled File Manager: drag a file row into the Composer and confirm exactly one chip is inserted
• pnpm --filter @pi-desktop/shared build
• pnpm --filter @pi-desktop/desktop typecheck
• node --experimental-strip-types --test test/composer-drag-drop.test.mjs (6 passed)
• node --test test/bundled-plugins.test.mjs (10 passed)
• pnpm lint
• pnpm docs:check
• pnpm build:js

[vastsa]

感谢时间，最近pr 有点多，我慢慢理！