Add a visual indicator to the field displays for 3-seconds after the match ends

**Is your feature request related to a problem? Please describe.**
Referees for Into the Deep are asked to assess PARKING or ASCENT points when all robots come to rest, or 3 seconds after the end of the match, whichever comes first (Section 10.5 in the Competition Manual). Section 9.10 states that "...audio cues are intended as a courtesy to participants and not intended as official MATCH markers.", so the length of the buzzer cannot be relied on for determining when 3 seconds after the end of the match is.

**Describe the solution you'd like**
Provide a clear/"loud" visual marker on the field display 3 seconds after the conclusion of the match. I don't have a strong opinion on the exact details, only that a referee who is primarily focusing on the SUBMERSIBLE/ASCENT RUNGS should be able to notice the difference out of the corner of their eyes. Something like the entire background changing from Black to White (or another equally contrasting change), or a pop-up over the time making it clear.

**Describe alternatives you've considered**
A couple of ideas I considered and then rejected

1. Notification on the referee tablets (akin to the yellow flashing when they need to progress to the next screen). Refs will be focused on the field, and shouldn't need to look down at their tablets for a timing indicator
2. Timer counting up after the match. I'm not sure this would be obvious enough to see without turning towards the monitor. Maybe in conjunction with a popup over the timer after 3 seconds?

**Additional context**
No other context at this point, happy to discuss this here or offline.

No files or tests are named. Start by locating the field-display entry point and the match-end timing state, then find existing display tests or run the relevant test suite. Done means a conspicuous visual marker appears on the field display three seconds after match conclusion and remains noticeable to referees.

Written by the indexing model from the issue text.