Assessment Editor: Fix dropdown positioning #294

Describe the bug
The @ mention autocomplete dropdown in the Assessment editor (step 3 → SUBMISSION prompt zone) has two positioning problems. The dropdown lives in app/components/assessment/editor/MentionDropdown.tsx.

1. Clipped at the bottom of the viewport. viewportPosition() limits left to the window width but never limits top. The list is max-h-52 (208px) with overflow-y-auto, so when the caret is near the bottom of the window the dropdown runs past the screen edge. Only the first ~3 rows can be reached and the rest cannot be scrolled to.
2. Drifts away from the caret on scroll. The position is computed once, when the dropdown opens. The dropdown is position: fixed, so scrolling the editor pane (overflow-y-auto in EditorStepLayout.tsx) moves the caret while the dropdown stays where it was.

To Reproduce

1. Open an assessor in the wizard and go to step 3 (Assessment).
2. In the SUBMISSION zone, type @ on a line near the bottom of the window → the dropdown is cut off and its lower rows can't be reached.
3. Type @ anywhere, then scroll the editor pane while the dropdown is open → the dropdown stays put and detaches from the text.

Expected behavior

• If there isn't enough room below the caret, the dropdown flips above the caret line (as app/components/ui/MultiSelect.tsx already does). In both positions the list fits on screen and scrolls.
• While open, the dropdown follows the caret when the editor scrolls or the window is resized.

Additional context
Should be a small change inside MentionDropdown.tsx: no new dependencies, and no changes to the caret math in useAtMention.ts.