Assessment UI: Fix display issues #290

Describe the bug
Three issues in the Assessment UI v2 (#289):

1. Dataset preview shows integers as decimals. Integer columns render as 12.0 in the data view modal and in the prompt editor's row-1 preview.
2. Overlapping marker in the prompt editor. While typing a column name after @, the required/optional ? badge renders on the half-typed token and sits over the caret and following text.
3. Mention dropdown stays open after backspace. After picking a column (or opening the ? marker), deleting the token with backspace leaves the type/required dropdown open.
4. Mention dropdown clipped by the card. The suggestion box is cut off at the bottom edge of the prompt card when the caret is near the bottom, because the card and its scroll container set overflow: hidden.
5. Column headers with spaces break @-mentions. Uploaded CSV/Excel headers like Student Name are sent to the backend as-is; the mention tokeniser expects contiguous names, so such columns can't be referenced cleanly.

To Reproduce

1. Assessment → New assessor → upload a sheet with an integer column and a header like Student Name
2. Click View on the submission set — integer cells show .0
3. Go to the Assessment step, type @Stud — a ? badge appears over the text before the column resolves
4. Pick a column, then backspace the whole token — dropdown stays open
5. Type @ on the last line of a zone — dropdown is cut off by the card edge
6. Try to reference Student Name — token does not resolve

Expected behavior

1. 12.0 renders as 12; real decimals like 12.5 unchanged.
2. Marker appears only once the token matches a known column.
3. Dropdown closes as soon as the token is edited.
4. Dropdown renders above the card, never clipped.
5. Headers are normalised before upload (Student Name → StudentName) for both CSV and Excel.

Screenshots
Marker overlap: @studen|? with dropdown below (see PR).

Desktop (please complete the following information):

• OS: macOS
• Browser: Chrome
• Version: current

Additional context
Branch: fix/assessment-v2-bugs, based on chore/sync-dev-to-main. PR: #288.