feat: H shows what's hidden; Spring keys that don't look secret are hidden

Sources, files and secrets get a Hidden flag. The repo provider marks
Spring keys that match neither pattern, files with only those, and repos
with only such files, instead of leaving them out.

- Hidden items are left out of the panes, Go to and lazysecrets list;
  a pane's counter says how many it hides.
- H (lists.toggleHidden) shows them, dimmed whole in mutedColor;
  list --hidden lists them, json marks them "hidden".
- A revealed empty value shows "∅ no value (an empty string)", muted,
  instead of a blank line.

internal/ui/panes/detail_test.go

@@ -3,6 +3,12 @@ package panes
 import (
 	"strings"
 	"testing"
+
+	"github.com/charmbracelet/x/ansi"
+
+	"git.kebcloud.no/keb/lazysecrets/internal/provider"
+	"git.kebcloud.no/keb/lazysecrets/internal/ui/icons"
+	"git.kebcloud.no/keb/lazysecrets/internal/ui/styles"
 )
 
 func TestWrappedValueKeepsItsStyle(t *testing.T) {
@@ -15,3 +21,17 @@ func TestWrappedValueKeepsItsStyle(t *testing.T) {
 		t.Errorf("the style went on past its reset: %q", got[2])
 	}
 }
+
+func TestEmptyValue(t *testing.T) {
+	d := NewDetail(icons.None)
+	d.SetSize(80, 20)
+	d.ShowSecret(provider.Object{Name: "spring.datasource.password", Type: provider.TypeSecret})
+	d.SetValue(provider.NewSecretValue(nil))
+	if view := ansi.Strip(d.View(styles.Default(), true, "")); !strings.Contains(view, "∅ no value (an empty string)") {
+		t.Errorf("empty value:\n%s", view)
+	}
+	d.SetValue(provider.NewSecretValue([]byte("empty")))
+	if view := ansi.Strip(d.View(styles.Default(), true, "")); strings.Contains(view, "∅") || !strings.Contains(view, "empty") {
+		t.Errorf("the word empty:\n%s", view)
+	}
+}