Autosave provider-backed drafts

[floklein]

Summary

• Add provider-backed draft types, IPC/preload/api bridge methods, and MailService draft orchestration.
• Implement Microsoft Graph and Gmail draft create/update/list/get/delete/send primitives.
• Autosave composer edits to provider drafts, show save state, keep saved drafts on close unless explicitly discarded, and send through provider draft APIs when available.

Validation

• C:\nvm4w\nodejs\npm.cmd --workspace @courrier/desktop test -- src/test/main/graph-client.test.ts src/test/main/gmail-client.test.ts src/test/lib/api-client.test.ts src/test/lib/mail/mail-query-options.test.ts src/test/lib/compose-window.test.ts
• C:\nvm4w\nodejs\npm.cmd run typecheck
• C:\nvm4w\nodejs\npm.cmd run lint
• C:\nvm4w\nodejs\npm.cmd test
• git diff --check