feat(2fa): show a scannable QR code when enabling two-factor auth

The setup screen showed the secret and the otpauth URI as text and asked
you to scan — with nothing to scan. It renders the URI as a QR now, an SVG
(one dark path on a white card, so it reads in either theme) generated from
the barcode encoder already in the module graph via pquerna/otp. The secret
stays below it for a device that cannot scan.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

go.mod

@@ -3,8 +3,10 @@ module github.com/kristianwind/verdande
 go 1.26.3
 
 require (
+	github.com/boombuler/barcode v1.0.1-0.20190219062509-6c824513bacc
 	github.com/coder/websocket v1.8.15
 	github.com/emersion/go-imap/v2 v2.0.0-beta.8
+	github.com/emersion/go-message v0.18.2
 	github.com/go-chi/chi/v5 v5.3.1
 	github.com/go-webauthn/webauthn v0.17.4
 	github.com/pquerna/otp v1.5.0
@@ -14,9 +16,7 @@ require (
 )
 
 require (
-	github.com/boombuler/barcode v1.0.1-0.20190219062509-6c824513bacc // indirect
 	github.com/dustin/go-humanize v1.0.1 // indirect
-	github.com/emersion/go-message v0.18.2 // indirect
 	github.com/emersion/go-sasl v0.0.0-20241020182733-b788ff22d5a6 // indirect
 	github.com/fxamacker/cbor/v2 v2.9.2 // indirect
 	github.com/go-viper/mapstructure/v2 v2.5.0 // indirect