Add bulk message selection

[floklein]

Summary

• Add multi-message selection in the message list with modifier clicks, range selection, keyboard navigation, select-all, Escape clear, Delete, and archive shortcuts.
• Add a compact bulk toolbar for read/archive/delete actions.
• Extend drag payloads and previews so dragging a selected row moves the full selected set to dropped folders.
• Add @tanstack/react-hotkeys dependency requested by the plan for shortcut work.

Validation

• C:\nvm4w\nodejs\npm.cmd --workspace @courrier/desktop test -- src/test/lib/mail/mail-drag.test.ts src/test/ui/MessageList.test.tsx
• C:\nvm4w\nodejs\npm.cmd run typecheck
• C:\nvm4w\nodejs\npm.cmd run lint
• C:\nvm4w\nodejs\npm.cmd test
• git diff --check