chore: modernize Go 1.26, update deps, fix lint issues

- Update Go version from 1.19 to 1.26
- Update all dependencies to latest versions
- Fix staticcheck warnings (unused vars, types, functions)
- Remove unused svgToPng function
- Add GitHub Actions CI workflow

graph/common/common.go

@@ -3,13 +3,11 @@ package common
 import (
 	"bytes"
 	"image/color"
-	"sync"
 
 	sc "github.com/taigrr/simplecolorpalettes/simplecolor"
 )
 
 var (
-	colorsLoaded sync.Once
 	colorScheme  []sc.SimpleColor
 )
 
@@ -19,8 +17,7 @@ func CreateGraph() bytes.Buffer {
 }
 
 func init() {
-	colors := []string{"#767960", "#a7297f", "#e8ca89", "#f5efd6", "#158266"}
-	colors = []string{"#0e4429", "#006d32", "#26a641", "#39d353"}
+	colors := []string{"#0e4429", "#006d32", "#26a641", "#39d353"}
 	for _, c := range colors {
 		color := sc.FromHexString(c)
 		colorScheme = append(colorScheme, color)