deps: migrate to xsync v4 + drop pkg/errors; bump protocol

[paulwe]

Summary

Bumps livekit/protocol to its latest tip (6b0178b3af2a) and picks up the protocol-side major dependency moves. This repo also drops its own pkg/errors usage and unpins from xsync v3 transitively.

• golang-jwt/jwt v5 (replacing go-jose/v4)
• xsync v3 -> v4
• pkg/errors -> stdlib (fmt.Errorf with %w)
• go.uber.org/atomic -> stdlib sync/atomic

Code changes

• pkg/rtcconfig/ip.go: dropped github.com/pkg/errors; errors.Errorf -> fmt.Errorf (with %w).

Module bumps

• livekit/protocol -> 6b0178b3af2a
• pion/ice/v4 4.2.6 -> 4.2.7, pion/webrtc/v4 4.2.3 -> 4.2.14, pion/sctp 1.9.2 -> 1.10.0, pion/turn/v5 5.0.4 -> 5.0.8, etc.
• go 1.24 -> go 1.26

Test plan

• go mod tidy + go build ./... clean
• go vet ./... clean
• No pion/transport v3/v4 mismatch (protocol bump aligns on v4)
• CI green