feat(tui): add the lazygit-style three-pane interface

Topics, messages, and message detail in three panes, with filtering, publishing, adding topics, a raw JSON view, history restore, and open-link support.

internal/tui/url.go

@@ -0,0 +1,14 @@
+package tui
+
+import (
+	"fmt"
+	"net/url"
+)
+
+func validateURL(raw string) error {
+	u, err := url.Parse(raw)
+	if err != nil || u.Host == "" || (u.Scheme != "https" && u.Scheme != "http") {
+		return fmt.Errorf("only http(s) links can be opened")
+	}
+	return nil
+}