feat(compose): add empty drafts action

[keilogic]

Fixes #534.

Summary

• Add an Empty Drafts icon button to the Draft Desk header.
• Confirm before deleting saved drafts, and disable the action while deletion is in progress.
• Batch-delete saved Drafts message IDs through the existing deleteMessages API path, then remove the deleted draft models locally.
• Keep unsaved local compose panes out of the backend delete request.
• Add Draft Desk service specs for the batch-delete behavior and the unsaved-only guard.

Validation

• npm ci
• npx tsc -p src/tsconfig.app.json --noEmit
• npx tsc -p src/tsconfig.spec.json --noEmit
• npx eslint src/app/compose/draftdesk.component.ts src/app/compose/draftdesk.service.ts src/app/compose/draftdesk.service.spec.ts src/app/compose/compose.module.ts
• npx ng test --watch=false --browsers=FirefoxHeadless --include=src/app/compose/draftdesk.service.spec.ts
• npm run lint -- --quiet
• npx ng test --watch=false --browsers=FirefoxHeadless
• node src/build/pre-build.js; npx ng build --configuration production --base-href=/app/ runbox7; node src/build/post-build.js
• npm run policy
• git diff --check
• git show --stat --oneline --check HEAD

Notes: the production build passed after fetching full git history for the changelog prebuild step. The build/test output included existing Angular Material/test-console/optimization warnings.

AI disclosure

This pull request was prepared with assistance from OpenAI Codex in the local working copy. Codex assisted with code changes, test updates, duplicate checks, and validation command execution.