feat(ui): 统一应用主题色为天空蓝并优化渐变背景

将应用主色调从 '#BBF246' 更改为 '#87CEEB'（天空蓝），并更新所有相关组件和页面中的颜色引用。同时为多个页面添加统一的渐变背景，提升视觉效果和用户体验。新增压力分析模态框组件，并优化压力计组件的交互与显示逻辑。更新应用图标和启动图资源。

app/challenge/index.tsx

@@ -1,4 +1,5 @@
 import { HeaderBar } from '@/components/ui/HeaderBar';
+import { Colors } from '@/constants/Colors';
 import { useAppDispatch, useAppSelector } from '@/hooks/redux';
 import { useAuthGuard } from '@/hooks/useAuthGuard';
 import { initChallenge } from '@/store/challengeSlice';
@@ -113,7 +114,7 @@ const styles = StyleSheet.create({
   },
   summaryLeft: { flexDirection: 'row', alignItems: 'center' },
   progressPill: { width: 120, height: 10, borderRadius: 999, backgroundColor: '#E5E7EB', overflow: 'hidden' },
-  progressFill: { height: '100%', backgroundColor: '#BBF246' },
+  progressFill: { height: '100%', backgroundColor: Colors.light.accentGreen },
   progressText: { marginLeft: 12, fontWeight: '700', color: '#111827' },
   summaryRight: {},
   summaryItem: { fontSize: 12, color: '#6B7280' },
@@ -133,7 +134,7 @@ const styles = StyleSheet.create({
   dayNumberLocked: { color: '#9CA3AF' },
   dayMinutes: { marginTop: 4, fontSize: 12, color: '#6B7280' },
   bottomBar: { padding: 20 },
-  startButton: { backgroundColor: '#BBF246', paddingVertical: 14, borderRadius: 999, alignItems: 'center' },
+  startButton: { backgroundColor: Colors.light.accentGreen, paddingVertical: 14, borderRadius: 999, alignItems: 'center' },
   startButtonText: { color: '#192126', fontWeight: '800', fontSize: 16 },
 });