Add stackable toast notifications

Replaces the single-slot status-line flash with a top-right toast
stack over the focused pane. flashError, flashTransient, and
notifyAttention all push onto the same stack (cap 5, FIFO drop).
Ctrl-N dismisses the most recent toast; empty stack falls through to
the focused PTY so readline / nano / emacs / opencode bindings keep
working. A new "Clear notifications" palette item empties the stack.

internal/app/palette.go

@@ -353,6 +353,12 @@ func (p *paletteState) buildItems(macro string) []paletteItem {
 		action: paletteAction{kind: "settings-open"},
 		group:  groupSettings,
 	})
+	out = append(out, paletteItem{
+		label:  "Clear notifications",
+		hint:   "dismiss all toasts in the top-right of the focused pane",
+		action: paletteAction{kind: "toasts-clear"},
+		group:  groupSettings,
+	})
 
 	// Group 4: Quit.
 	out = append(out, paletteItem{