Fix chart display and simplify predict page UI

Fixed issues:
1. Chart display error - Fixed JavaScript template literals that were broken when $ was replaced with CHF
   - Changed `chart_CHF {watch.id}` to `chart_${watch.id}` in historical.html
   - Fixed all template literals in predict.html

2. Simplified predict page UI:
   - Added Excel column paste functionality - students can click on watch headers and paste 12 values
   - Removed unnecessary buttons (Fill Increasing, Fill Decreasing, Clear All)
   - Kept only "Fill Constant" and "Fill with Last Year" buttons
   - Updated instructions to explain paste functionality

🤖 Generated with [Claude Code](https://claude.com/claude-code

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