fix(mail): show empty state for unread-only folders

[SimoneMariaRomeo]

Closes #689

Summary:

• Show a clear empty-state notice when Unread only leaves the current folder with no visible messages.
• Keep the canvas table intact and render the notice as a lightweight overlay.
• Add a focused helper spec so the empty-state behavior is covered separately from the Angular app component.

Testing:

• npm run test -- --watch=false --browsers=Firefox --include=src/app/message-list-empty-notice.spec.ts
• npm run policy
• npm run lint (passes with existing warnings)
• npx ng build --base-href=/app/ runbox7 (passes with existing warnings)
• node run-ci-tests.js unit (fails in existing component-test setup with Angular Material unknown-element errors outside this change)

AI disclosure: I used OpenAI Codex locally to inspect the issue, implement the patch, and run validation commands.