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 <noreply@anthropic.com>

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

internal/auth/totp.go

@@ -10,6 +10,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/boombuler/barcode/qr"
 	"github.com/pquerna/otp"
 	"github.com/pquerna/otp/totp"
 )
@@ -50,6 +51,41 @@ func NewTOTPSecret(issuer, account string) (secret, uri string, err error) {
 	return key.Secret(), key.URL(), nil
 }
 
+// TOTPQRSVG renders the otpauth:// URI as a QR code, as an SVG a phone camera can
+// read. SVG rather than a PNG data URI so it stays crisp at any size and adds no
+// base64 blob to the page — one dark path on a white square, which is what a scanner
+// needs whatever theme the page is in.
+//
+// The barcode encoder is already in the module graph (pquerna/otp draws on it), so
+// this borrows a dependency rather than adding one.
+func TOTPQRSVG(uri string) (string, error) {
+	code, err := qr.Encode(uri, qr.M, qr.Auto)
+	if err != nil {
+		return "", fmt.Errorf("auth: encode totp qr: %w", err)
+	}
+	b := code.Bounds()
+	n := b.Dx()
+	// A four-module quiet zone all round, which the spec requires and scanners rely
+	// on — without it the code runs to the edge and a reader cannot find its border.
+	const quiet = 4
+	size := n + quiet*2
+
+	var sb strings.Builder
+	fmt.Fprintf(&sb, `<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 %d %d" shape-rendering="crispEdges">`, size, size)
+	sb.WriteString(`<rect width="100%" height="100%" fill="#ffffff"/><path fill="#111312" d="`)
+	for y := 0; y < n; y++ {
+		for x := 0; x < n; x++ {
+			// Dark modules are black; the red channel is 0 for those and full for the
+			// light ones. One 1×1 rect per dark module, merged into a single path.
+			if r, _, _, _ := code.At(b.Min.X+x, b.Min.Y+y).RGBA(); r == 0 {
+				fmt.Fprintf(&sb, "M%d %dh1v1h-1z", x+quiet, y+quiet)
+			}
+		}
+	}
+	sb.WriteString(`"/></svg>`)
+	return sb.String(), nil
+}
+
 // TOTPURI rebuilds the otpauth:// URI for an existing secret, for showing the QR
 // code again before enrolment is confirmed.
 func TOTPURI(secret, issuer, account string) string {

internal/httpapi/totp_handlers.go

@@ -17,6 +17,9 @@ import (
 type totpSetupResponse struct {
 	Secret string `json:"secret"`
 	URI    string `json:"uri"`
+	// The URI as a scannable QR code (SVG). The secret and URI stay for typing in by
+	// hand on a device that cannot scan.
+	QR string `json:"qr"`
 }
 
 func (s *Server) handleTOTPSetup(w http.ResponseWriter, r *http.Request) {
@@ -54,7 +57,12 @@ func (s *Server) handleTOTPSetup(w http.ResponseWriter, r *http.Request) {
 		s.internal(w, r, "store totp secret", err)
 		return
 	}
-	writeJSON(w, http.StatusOK, totpSetupResponse{Secret: secret, URI: uri})
+	qrSVG, err := auth.TOTPQRSVG(uri)
+	if err != nil {
+		s.internal(w, r, "render totp qr", err)
+		return
+	}
+	writeJSON(w, http.StatusOK, totpSetupResponse{Secret: secret, URI: uri, QR: qrSVG})
 }
 
 type totpConfirmResponse struct {

web/src/routes/indstillinger/+page.svelte

@@ -138,6 +138,7 @@
 
 	let totpSecret = $state('');
 	let totpURI = $state('');
+	let totpQR = $state('');
 	let totpCode = $state('');
 	let totpErrors = $state({});
 	let recoveryCodes = $state([]);
@@ -163,6 +164,7 @@
 			enablePassword = '';
 			totpSecret = r.secret;
 			totpURI = r.uri;
+			totpQR = r.qr;
 		} catch (e) {
 			totpErrors = e.fields ?? {};
 			if (!Object.keys(totpErrors).length) app.toast(humanMessage(e));
@@ -177,6 +179,7 @@
 			recoveryCodes = r.recovery_codes;
 			totpSecret = '';
 			totpURI = '';
+			totpQR = '';
 			totpCode = '';
 			app.user = await api.me();
 		} catch (e) {
@@ -492,8 +495,13 @@
 	{:else if totpSecret}
 		<div class="field">
 			<p class="hint">{t('account.scanHint')}</p>
+			{#if totpQR}
+				<!-- The server renders the otpauth URI as an SVG QR — one dark path on a
+				     white square, so it scans in either theme. It is the URI it already
+				     sent, drawn; nothing here builds it. -->
+				<div class="qr">{@html totpQR}</div>
+			{/if}
 			<p class="mono secret">{totpSecret}</p>
-			<p class="hint mono uri">{totpURI}</p>
 		</div>
 
 		<form onsubmit={confirmTOTP}>
@@ -514,7 +522,14 @@
 
 			<div class="row">
 				<button class="primary" type="submit">{t('account.confirm')}</button>
-				<button class="secondary" onclick={() => (totpSecret = '')}>{t('account.cancel')}</button>
+				<button
+				class="secondary"
+				onclick={() => {
+					totpSecret = '';
+					totpURI = '';
+					totpQR = '';
+				}}>{t('account.cancel')}</button
+			>
 			</div>
 		</form>
 	{:else}
@@ -859,6 +874,23 @@
 		color: var(--ink-faint);
 	}
 
+	/* On its own white card whatever the theme: a QR is dark-on-light or it does not
+	   scan, so the square keeps its own background rather than taking the page's. */
+	.qr {
+		width: 176px;
+		height: 176px;
+		padding: var(--s2);
+		background: #ffffff;
+		border: 1px solid var(--line);
+		border-radius: var(--radius);
+	}
+
+	.qr :global(svg) {
+		display: block;
+		width: 100%;
+		height: 100%;
+	}
+
 	.secret {
 		margin: 0;
 		padding: var(--s3);
@@ -869,10 +901,6 @@
 		overflow-wrap: anywhere;
 	}
 
-	.uri {
-		overflow-wrap: anywhere;
-	}
-
 	/* The understated variant: present, not loud. Bordered and in the muted ink so
 	   it reads as a footnote, brightening on hover. The cup is outline-only. */
 	.bmc {