feat: add vikunja doctor command for diagnostic checks (#2165)

Add a new `vikunja doctor` CLI command that performs diagnostic checks.

Checks performed:

- **System**: Version, Go version, OS/arch, running user, working
directory
- **Configuration**: Config file path, public URL, JWT secret, CORS
origins
- **Database**: Connection test, server version
(SQLite/MySQL/PostgreSQL)
- **Files**: Storage path, writability, disk space (Unix only)
- **Optional services** (when enabled):
  - Redis: Connection ping
  - Typesense: Health endpoint
  - Mailer: SMTP connection
  - LDAP: Bind authentication test
  - OpenID Connect: Discovery endpoint for each configured provider

pkg/doctor/system_unix.go

@@ -0,0 +1,40 @@
+//go:build !windows
+
+// 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 doctor
+
+import (
+	"fmt"
+	"os"
+	osuser "os/user"
+)
+
+func checkUser() CheckResult {
+	uid := os.Getuid()
+	username := "unknown"
+
+	if u, err := osuser.Current(); err == nil {
+		username = u.Username
+	}
+
+	return CheckResult{
+		Name:   "User",
+		Passed: true,
+		Value:  fmt.Sprintf("%s (uid=%d)", username, uid),
+	}
+}