WIP: grading-process walkthrough + temp-dir cleanup paused (NOT FINISHED)

[theRealProHacker]

⚠️ Work in progress — do not merge as-is. This PR backs up pending local work so it isn't lost, but two things are unfinished.

What's in this PR (3 files, +7 / −4)

• README.md — started a new "Prozess" (grading walkthrough) section, broadened the intro to "FU Berlin Whiteboard tutors", removed the stale Windows stable-channel TextField warning.
• images/assigments-grade.png (new) — screenshot referenced by the walkthrough.
• lib/logic.dart — commented out the post-zip temp-directory cleanup.

❌ What still needs to be done

1. Finish the README "Prozess" walkthrough. It ends with a dangling empty bullet and only covers the first step:

   Prozess:
   - Im Whiteboard bei dem jeweiligen Assignment auf Grade drücken
       ![...](images/assigments-grade.png)
   -          ← empty bullet, section is cut off here
   

   Add the remaining steps (open in Tut-Tool → review/comment by group → submit grades back to grades.csv).

2. Restore the temp-directory cleanup in lib/logic.dart (around line 239). It was commented out, likely for debugging:

   // await copiedDir.delete(recursive: true);

   As-is, the temporary copied directory is never deleted after zipping, so temp dirs accumulate on every export. Either un-comment it or replace it with the intended cleanup behavior before release.

3. Confirm the removed Windows warning is actually obsolete. The old README warned that Flutter's stable channel had a TextField bug requiring beta/master. Verify that's no longer true before dropping the note for good.

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