fix: TypeScript null safety fixes for strict mode

- RateAnalysis: null guard on price_incl in timeline trace
- Layout: explicit null check on alertCount, use ?? for user.name

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/pages/RateAnalysis.tsx

@@ -354,7 +354,7 @@ function buildTimelineTraces(entries: TimelineEntry[]) {
     mode: 'lines+markers' as const,
     name: room,
     x: pts.map(p => p.scraped_at),
-    y: pts.map(p => p.price_incl),
+    y: pts.map(p => p.price_incl ?? null),
     line: { color: colors[i % colors.length], width: 2 },
     marker: { size: 5, color: colors[i % colors.length] },
     hovertemplate: `${room}: £%{y:.2f}<extra></extra>`,