CUB-196: fix future timestamps, negative battery clamp, boundary tests

Grimm review fixes:
- formatRelativeTime: guard future timestamps (clock skew) → 'unknown'
- battery display: clamp negative values to 0%
- formatTimeLeft: floor fractional seconds
- Tests: +4 (future timestamp, negative battery, 15%/50% boundaries)

src/components/CameraCard.tsx

@@ -8,6 +8,7 @@ function formatRelativeTime(iso: string): string {
   if (isNaN(then)) return 'unknown'
 
   const diffSec = Math.floor((Date.now() - then) / 1000)
+  if (diffSec < 0) return 'unknown'
 
   if (diffSec < 10) return 'just now'
   if (diffSec < 60) return `${diffSec}s ago`
@@ -32,7 +33,7 @@ function batteryColor(pct: number | null): { bar: string; text: string } {
 function formatTimeLeft(sec: number): string {
   if (sec <= 0 || !isFinite(sec)) return '--'
   const m = Math.floor(sec / 60)
-  const s = sec % 60
+  const s = Math.floor(sec % 60)
   return `${m}m ${s}s left`
 }
 
@@ -138,7 +139,7 @@ export default function CameraCard({ camera }: CameraCardProps) {
               Battery
             </span>
             <span className={`font-mono text-xs ${batt.text}`}>
-              {battery_pct !== null ? `${battery_pct}%` : 'N/A'}
+              {battery_pct !== null ? `${Math.max(0, battery_pct)}%` : 'N/A'}
             </span>
           </div>
           <div