feat: enhanced graph with color gradients, sparkline, trend, and peak marker

[simplynadaf]

What does this PR do?

Upgrades devpub stats --graph from basic single-color bars to a visually rich terminal chart with professional-grade enhancements.

Enhancements

Feature	Description
Color gradient bars	Bars fade from blue (low) to cyan to green to gold (peak) based on value
Sparkline one-liner	Compact trend: line below chart for quick pattern recognition
Trend indicator	Arrow + percentage comparing last 7 days vs previous 7 days
Average line	Dotted line across chart showing mean, with label
Peak marker	Bold gold peak value with date in summary
Box-drawing axes	Proper tick marks with Unicode box characters
12-row height	More vertical resolution (was 10)
4 Y-axis labels	Shows 25%, 50%, 75%, 100% marks (was 3)

Example output

  Views (last 7 days)  +23%

     770 |    X
         |    X
         | X  XX
     577 | X  XXX
         | XX.XXX. <- avg (532)
         |XXX XXX
     385 |XXX XXXX
         |XXXXXXXX
         |XXXXXXXX
     192 |XXXXXXXX
         |XXXXXXXX
         |XXXXXXXX
         ----------
          Aug 13 Aug 17 Aug 20

  trend: .......

  Total: 4.3K | Avg: 532/day | Peak: 770 ^ Aug 17

(With Rich colors: gradient blue to gold, avg line in dim)

Zero new dependencies

All rendering uses Rich (already a dependency) and pure Python. No external charting libraries added.

Tests

41 tests passing (18 new tests for color, sparkline, and trend helpers):

pytest tests/test_analytics.py -v
41 passed in 0.07s

Checklist

• Tests pass (pytest)
• No em dashes used anywhere
• New functionality has tests
• Zero new dependencies
• Backwards compatible (same CLI interface)