Add XP on the adventurer page

[fboucher]

Summary

Add a dedicated "Add XP" widget to the Adventurer sheet so the dungeon master or player can quickly award a specific amount of XP without manually editing the total.

Current Behaviour

The XP field on AdventurerCard.razor is a single FluentNumberField (80px). Users must calculate and type the new total themselves.

Desired Behaviour

Keep the existing XP display/direct-edit field. Add a delta widget alongside it consisting of:

• A numeric input (positive integers only) labelled something like "Add XP"
• A "+Add" button that adds the entered delta to the current XP total

After clicking "+Add":

• The current XP total increases by the delta amount.
• The delta input field clears back to 0 automatically.
• No other visual feedback is needed — the updated XP total speaks for itself.

Constraints

• The delta input should only accept positive values (≥ 1). Negative values / subtraction are out of scope; users can correct mistakes via the existing direct-edit XP field.

Acceptance Criteria

• A numeric delta input and "+Add" button appear on the Adventurer sheet next to the XP field
• Clicking "+Add" adds the delta to the XP total
• The delta input resets to 0 (or empty) after each successful add
• The delta input rejects or ignores non-positive values
• The existing XP field still allows direct editing of the total

[fboucher]

done queued for the next version