Feedbacks refactor, comments, and style

backend/app/crud.py

backend/app/main.py

backend/app/models.py

backend/app/schemas.py

frontend/src/lib/api/sessions.ts

frontend/src/lib/components/icons/chatBubble.svelte

frontend/src/lib/components/icons/spellCheck.svelte

frontend/src/lib/components/sessions/message.svelte

frontend/src/lib/components/tests/typingtest.svelte

frontend/src/lib/types/feedback.ts

frontend/src/lib/types/message.ts

frontend/src/lib/types/session.ts

frontend/src/routes/tests/vocabulary/+page.svelte

frontend/src/routes/tutor/timeslots/+page.svelte