Fix portal timers stopping when tab is backgrounded + upgrade Next.js to patch DoS CVE

[Copilot]

Description

Portal timers (field/scoresheet and judging/rubrics) halt when the browser tab loses focus because browsers throttle setInterval in backgrounded tabs, causing ticks to be skipped entirely.

Timer fix (use-field-timer.ts, use-judging-timer.ts)

Replaced the tick-counting approach with wall-clock compensation:

• Track lastTickAt (wall-clock timestamp) via ref when the timer starts/resumes
• Poll every 500ms instead of 1000ms
• On each poll, compute elapsedTicks = Math.floor((now - lastTickAt) / 1000) and advance lastTickAt by only whole seconds — fractional time accumulates correctly across polls
• Dispatch a single TICK_BY action instead of a loop of TICK dispatches

intervalRef.current = setInterval(() => {
  if (lastTickAtRef.current === null) return;
  const now = Date.now();
  const elapsedMs = now - lastTickAtRef.current;
  const elapsedTicks = Math.floor(elapsedMs / 1000);
  if (elapsedTicks > 0) {
    lastTickAtRef.current += elapsedTicks * 1000;
    dispatch({ type: 'TICK_BY', count: elapsedTicks });
  }
}, 500);

TICK_BY in the judging timer handles multi-stage transitions in a single reducer call. The effect dependency is reduced to [state.isRunning] — no longer recreated on every tick.

The field timer's endgame sound uses a threshold-crossing check (prev > 30 && curr <= 30) instead of exact equality, so the 30-second cue plays even if time jumps past that mark while backgrounded.

Dependency upgrade (package.json, package-lock.json)

Upgraded next from ^16.1.6 → ^16.2.3 (resolves to 16.2.4) to patch a Denial of Service via Server Components vulnerability (affected >= 16.0.0-beta.0, < 16.2.3). Companion packages updated to match:

Package	Before	After
next	^16.1.6	^16.2.3
eslint-config-next	^16.1.6	^16.2.3
@next/eslint-plugin-next	^16.1.1	^16.2.3

Type of change

• Bug fix (non-breaking change which fixes an issue)

Screenshots

N/A — timer logic only, no UI changes.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• My changes generate no new warnings