Fix command prefix bug and audit issues across all new plugins

All new plugins used Base{Client: client} instead of NewBase(client),
leaving Prefix empty so no !commands matched. Also fixes hangman UTF-8
safety (use runes not byte indices), euro race conditions, blackjack
deck/timer bugs, and adds difficulty selection to hangman start.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

internal/plugin/flip.go

@@ -13,7 +13,7 @@ type FlipPlugin struct {
 }
 
 func NewFlipPlugin(client *mautrix.Client) *FlipPlugin {
-	return &FlipPlugin{Base: Base{Client: client}}
+	return &FlipPlugin{Base: NewBase(client)}
 }
 
 func (p *FlipPlugin) Name() string { return "flip" }