rename replay query parameter to initChannelState

Rename ?replay=1 to ?initChannelState=1 across server, SPA, and docs
per review feedback: the parameter initialises fresh channel state
rather than replaying past state.

- Rename replayChannelState() to initChannelState()
- Update query parameter check in HandleState
- Update SPA fetch URL and comments
- Update README documentation and curl examples

internal/handlers/auth.go

@@ -182,9 +182,9 @@ func (hdlr *Handlers) handleLogin(
 		request, clientID, sessionID, payload.Nick,
 	)
 
-	// Replay channel state so the new client knows which
+	// Init channel state so the new client knows which
 	// channels the session already belongs to.
-	hdlr.replayChannelState(
+	hdlr.initChannelState(
 		request, clientID, sessionID, payload.Nick,
 	)