Switch IMAP library from go-imap/v2 to v1 for Outlook compatibility

go-imap/v2's strict wire parser rejects Outlook's non-standard IMAP
login responses. v1 is more lenient and handles these gracefully.

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

backend/go.mod

@@ -5,7 +5,8 @@ go 1.25
 require (
 	github.com/99designs/gqlgen v0.17.88
 	github.com/anthropics/anthropic-sdk-go v1.26.0
-	github.com/emersion/go-imap/v2 v2.0.0-beta.8
+	github.com/emersion/go-imap v1.2.1
+	github.com/emersion/go-message v0.18.2
 	github.com/gin-gonic/gin v1.11.0
 	github.com/golang-jwt/jwt/v5 v5.3.0
 	github.com/gorilla/websocket v1.5.3
@@ -23,7 +24,6 @@ require (
 	github.com/bytedance/sonic v1.14.0 // indirect
 	github.com/bytedance/sonic/loader v0.3.0 // indirect
 	github.com/cloudwego/base64x v0.1.6 // indirect
-	github.com/emersion/go-message v0.18.2 // indirect
 	github.com/emersion/go-sasl v0.0.0-20241020182733-b788ff22d5a6 // indirect
 	github.com/gabriel-vasile/mimetype v1.4.8 // indirect
 	github.com/gin-contrib/sse v1.1.0 // indirect