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/keys/keys.go

@@ -45,6 +45,7 @@ type Map struct {
 	Recent                            key.Binding
 	Note, Share, ShareMenu, OpenRef   key.Binding
 	Search, SourcesView, Commands     key.Binding
+	ToggleHidden                      key.Binding
 	SplitLeft, SplitRight, SplitReset key.Binding
 	Taller, Shorter, ZoomIn, ZoomOut  key.Binding
 	Login, Refresh, Help, Quit        key.Binding
@@ -102,6 +103,7 @@ func (m *Map) Actions() []Action {
 		a(Lists, "prevTab", &m.PrevTab, "prev tab", "lists", "Previous tab of the focused pane: Sources and Favourites in Sources; Secrets, Keys, Certs in Secrets."),
 		a(Lists, "nextTab", &m.NextTab, "next tab", "lists", "Next tab of the focused pane."),
 		a(Lists, "sourcesView", &m.SourcesView, "grouped/flat sources", "lists", "Switch the Sources pane between grouped by provider and a flat list."),
+		a(Lists, "toggleHidden", &m.ToggleHidden, "show/hide hidden", "lists", "Show or hide what's hidden: Spring config keys that don't look like secrets, and files and repos with only those. Shown, they're dimmed; Go to finds them too."),
 		a(Lists, "star", &m.Favourite, "star", "lists", "Star or unstar the selected subscription, vault or secret, for the account showing it. Starred items get a ★ and sort first."),
 		a(Lists, "goTo", &m.Search, "search", "lists", "Go to anything: search every vault and secret loaded and jump to one. Again in Go to: only the source or group you're in, and back to all. On the left, the scope: All, an account (or Databricks, or Repos), or one source (○ ◐ ● for how much of it is loaded)."),
 		a(Lists, "favourites", &m.Favourites, "favourites", "lists", "Go to, on Favourites: everything you've starred, to search and jump to."),
@@ -185,7 +187,7 @@ var defaults = map[string][]string{
 	"lists.nextPane": {"tab"}, "lists.prevPane": {"shift+tab", "h", "left"}, "lists.details": {"l", "right"},
 	"lists.pane1": {"1"}, "lists.pane2": {"2"}, "lists.pane3": {"3"}, "lists.pane4": {"4"}, "lists.pane0": {"0"},
 	"lists.open": {"enter"}, "lists.back": {"esc", "backspace"},
-	"lists.filter": {"/"}, "lists.prevTab": {"["}, "lists.nextTab": {"]"}, "lists.sourcesView": {"`"},
+	"lists.filter": {"/"}, "lists.prevTab": {"["}, "lists.nextTab": {"]"}, "lists.sourcesView": {"`"}, "lists.toggleHidden": {"H"},
 	"lists.star": {"f"}, "lists.favourites": {"F"}, "lists.recent": {"R"}, "lists.goTo": {"ctrl+s"}, "lists.note": {"n"},
 	"lists.commands": {"x"},
 	"lists.reveal":   {"space"}, "lists.copyValue": {"y"}, "lists.copyReference": {"Y"},