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/term/term.go

@@ -3,7 +3,6 @@ package term
 import (
 	"os"
 	"strings"
-	"sync"
 
 	"github.com/muesli/termenv"
 	sc "github.com/taigrr/simplecolorpalettes/simplecolor"
@@ -11,11 +10,6 @@ import (
 	"github.com/taigrr/gico/graph/common"
 )
 
-var (
-	colorsLoaded sync.Once
-	colorScheme  []sc.SimpleColor
-)
-
 func GetWeekUnicode(frequencies []int) string {
 	squareColors := []sc.SimpleColor{}
 	min, max := common.MinMax(frequencies)