fix: resolve golangci-lint v2.1.6 staticcheck failures

- SA9003 (empty branch): rewrote TestFormatLargeAndScientific to actually
  call FormatForDisplay and perform a minimal non-empty assertion instead of
  an if-with-no-body.
- SA1019 (deprecated keyStyle.Copy()): replaced all .Copy() with direct
  assignment or method chaining on the base keyStyle. lipgloss styles are
  immutable and their builder methods already return new instances.
- Removed now-unused "strings" import from calc_test.go.
- All changes pass go test -short, go build, go vet, and gofmt.

This makes the Lint job (which runs golangci-lint-action + staticcheck) green.
The v0.3.0 tag was already pushed from the prior commit; this is master
hygiene so future CI + any follow-up tags are clean.

internal/calc/calc_test.go

@@ -2,7 +2,6 @@ package calc
 
 import (
 	"errors"
-	"strings"
 	"testing"
 )
 
@@ -240,8 +239,11 @@ func TestNegativeInBases(t *testing.T) {
 func TestFormatLargeAndScientific(t *testing.T) {
 	e := NewEngine()
 	e.accumulator = 1e15
-	if got := e.FormatForDisplay(); !strings.Contains(got, "e") && !strings.Contains(got, "E") {
-		// may or not, but check no panic
+	got := e.FormatForDisplay()
+	// Large values may or may not use scientific notation (depending on FormatForDisplay
+	// rules and current base); the primary purpose of this test is to ensure no panic.
+	if got == "" {
+		t.Error("FormatForDisplay returned empty for large accumulator value")
 	}
 }
 

internal/ui/ui.go

@@ -230,11 +230,11 @@ func (a *App) View() string {
 		Align(lipgloss.Center)
 
 	// Specialized key styles for visual grouping (like real calculators).
-	numKey := keyStyle.Copy()
-	opKey := keyStyle.Copy().Foreground(lipgloss.Color("63")).Background(lipgloss.Color("235"))
-	clearKey := keyStyle.Copy().Foreground(lipgloss.Color("203")).Background(lipgloss.Color("52"))
-	modKey := keyStyle.Copy().Foreground(lipgloss.Color("214"))                                   // orange-ish for MOD
-	hexKey := keyStyle.Copy().Foreground(lipgloss.Color("214")).Background(lipgloss.Color("235")) // for A-F, only shown in HEX mode
+	numKey := keyStyle
+	opKey := keyStyle.Foreground(lipgloss.Color("63")).Background(lipgloss.Color("235"))
+	clearKey := keyStyle.Foreground(lipgloss.Color("203")).Background(lipgloss.Color("52"))
+	modKey := keyStyle.Foreground(lipgloss.Color("214"))                                   // orange-ish for MOD
+	hexKey := keyStyle.Foreground(lipgloss.Color("214")).Background(lipgloss.Color("235")) // for A-F, only shown in HEX mode
 
 	makeKey := func(label string) string {
 		var st lipgloss.Style