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.go

@@ -335,6 +335,12 @@ func (d Detail) lines(st styles.Styles) []string {
 			}
 		}
 	case ValueShown:
+		if d.value.Reveal() == "" {
+			// Not in the value's colour, nor a word a value could be.
+			field(label, st.Muted.Italic(true).Render("∅ no value (an empty string)"))
+			out = append(out, valueIndent+hints("hide"))
+			break
+		}
 		// Where the dots were, continuing under itself; the keys below.
 		first := true
 		lines, note := valueView(d.value.Reveal())