feat: add OAuth client validation and PKCE verification

Add redirect URI validation that allowlists vikunja-* custom protocol
schemes, rejecting http/https and dangerous schemes like javascript:.
Add PKCE S256 verification following RFC 7636.

pkg/modules/auth/oauth2server/client.go

@@ -0,0 +1,35 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package oauth2server
+
+import (
+	"net/url"
+	"strings"
+)
+
+// ValidateRedirectURI checks that the redirect_uri uses a scheme starting with
+// "vikunja-". This allowlists only Vikunja native app schemes (e.g.
+// vikunja-flutter://callback) and rejects dangerous schemes like javascript:,
+// data:, http:, https:, etc.
+func ValidateRedirectURI(redirectURI string) bool {
+	u, err := url.Parse(redirectURI)
+	if err != nil || u.Scheme == "" {
+		return false
+	}
+
+	return strings.HasPrefix(u.Scheme, "vikunja-")
+}

pkg/modules/auth/oauth2server/pkce.go

@@ -0,0 +1,34 @@
+// Vikunja is a to-do list application to facilitate your life.
+// Copyright 2018-present Vikunja and contributors. All rights reserved.
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Affero General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU Affero General Public License for more details.
+//
+// You should have received a copy of the GNU Affero General Public License
+// along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+package oauth2server
+
+import (
+	"crypto/sha256"
+	"encoding/base64"
+)
+
+// VerifyPKCE verifies a PKCE code_verifier against a stored code_challenge using S256.
+// S256: BASE64URL(SHA256(code_verifier)) == code_challenge
+func VerifyPKCE(codeVerifier, codeChallenge, codeChallengeMethod string) bool {
+	if codeChallengeMethod != "S256" {
+		return false
+	}
+
+	h := sha256.Sum256([]byte(codeVerifier))
+	computed := base64.RawURLEncoding.EncodeToString(h[:])
+	return computed == codeChallenge
+}