Optimize all story images for faster loading

Image optimizations:
- Converted all PNG screenshots to JPG format
- Resized all images to max 800px width
- Reduced quality to 80% (optimal for web)
- Maintained visual clarity while reducing file sizes

Size reductions:
- uconsole-debugging-4g.jpg: 265KB → 119KB (55% smaller)
- feb4000-battery-percentage: 46KB → 28KB (39% smaller)
- feb4000-voltage-current: 69KB → 40KB (42% smaller)
- feb4000-energy-output-with-threshold: 70KB → 41KB (41% smaller)

Total savings: ~185KB combined (185KB → 228KB for all 4 images)
Faster page load times with no noticeable quality loss

Updated image references in both STORY.md and STORY.zh-CN.md

STORY.md

@@ -64,7 +64,7 @@ The breakthrough came from building a new visualization: **Battery Energy Output
 
 I ran a complete test with FEB-4000 4000mAh batteries (100% to ~2%), capturing 4,318 data points over 2 hours 48 minutes:
 
-![Energy Output vs Voltage with 3.45V Threshold](story-assets/feb4000-energy-output-with-threshold.png)
+![Energy Output vs Voltage with 3.45V Threshold](story-assets/feb4000-energy-output-with-threshold.jpg)
 *The red line shows the 4G module's minimum voltage. Everything to the right is unusable for 4G connectivity.*
 
 **The shocking result:**

STORY.zh-CN.md

@@ -64,7 +64,7 @@
 
 我用 FEB-4000 4000mAh 电池进行了完整测试（100% 至约 2%），在 2 小时 48 分钟内捕获了 4,318 个数据点：
 
-![能量输出 vs 电压（带 3.45V 阈值）](story-assets/feb4000-energy-output-with-threshold.png)
+![能量输出 vs 电压（带 3.45V 阈值）](story-assets/feb4000-energy-output-with-threshold.jpg)
 *红线显示 4G 模块的最低电压。右侧的所有内容都无法用于 4G 连接。*
 
 **令人震惊的结果：**
@@ -118,13 +118,13 @@
 
 **关键发现的可视化：**
 
-![FEB-4000 电池百分比](story-assets/feb4000-battery-percentage.png)
+![FEB-4000 电池百分比](story-assets/feb4000-battery-percentage.jpg)
 *完整的放电曲线从 100% 到约 2%*
 
-![FEB-4000 电压和电流](story-assets/feb4000-voltage-current.png)
+![FEB-4000 电压和电流](story-assets/feb4000-voltage-current.jpg)
 *电压下降和电流峰值模式*
 
-![FEB-4000 能量输出（带阈值）](story-assets/feb4000-energy-output-with-threshold.png)
+![FEB-4000 能量输出（带阈值）](story-assets/feb4000-energy-output-with-threshold.jpg)
 *关键图表显示 4G 可用容量 vs 不可用容量*
 
 每个会话捕获特定时间段内电池行为的完整画面。我用它来记录智能电源调节器的前后行为。