fix(oidc): clamp post_login_redirect to CORS allow-list (open-redirect)

Closes the open-redirect risk surfaced by the v0.3.x security audit:
the OIDC callback handler was using oidc_config.post_login_redirect
verbatim as the redirect base on both error and success paths. An
attacker with admin-key compromise (or a misconfigured operator) could
set that field to an external domain and turn /api/auth/oidc/callback
into a phishing redirector ('dialectic login failed → re-enter
password at evil.com/login').

Fix:
  - New AuthHandler.safeRedirectBase(raw) validator:
      * empty → '/'
      * relative path starting with '/' (but not '//') → keep as-is
      * absolute URL whose host is in the allow-list → keep as-is
      * everything else → '/'
  - allow-list sourced from cfg.CORSAllowOrigins (the same set we
    already trust for browser CORS), threaded through NewAuthHandler.
  - Applied on BOTH the error branch and success branch of Callback.

Also: drop redundant newline on cmd/dialectic-cli usage Fprintln so
go test ./... passes.

No behavior change for happy path: prod's PostLoginRedirect is
'https://dialectic.hangman-lab.top/oidc/callback', which matches the
CORS allow-list ('https://dialectic.hangman-lab.top'), so the
validator returns it unmodified.

internal/httpapi/handlers/auth.go

@@ -21,16 +21,56 @@ import (
 //   GET  /api/auth/me               — current session user (401 if anon)
 //   POST /api/auth/logout           — clears the session cookie
 type AuthHandler struct {
-	oidc       *oidc.Service
-	cookieName string
-	secure     bool
+	oidc        *oidc.Service
+	cookieName  string
+	secure      bool
+	allowedHosts []string // for open-redirect protection on post_login_redirect
 }
 
-func NewAuthHandler(svc *oidc.Service, cookieName string, secure bool) *AuthHandler {
+// NewAuthHandler wires the OIDC HTTP surface. `allowedHosts` is the
+// allow-list of hostnames the callback may 302 the browser to (sourced
+// from cfg.CORSAllowOrigins — the same origins we already trust for
+// CORS, by definition. A relative path always passes regardless.
+func NewAuthHandler(svc *oidc.Service, cookieName string, secure bool, allowedHosts []string) *AuthHandler {
 	if cookieName == "" {
 		cookieName = "dialectic_session"
 	}
-	return &AuthHandler{oidc: svc, cookieName: cookieName, secure: secure}
+	return &AuthHandler{oidc: svc, cookieName: cookieName, secure: secure, allowedHosts: allowedHosts}
+}
+
+// safeRedirectBase returns `raw` if it's a relative path OR an absolute
+// URL whose host appears in the allow-list. Otherwise it returns "/" so
+// the open-redirect attack (attacker-set PostLoginRedirect points to
+// evil.com) is neutered — the worst the SPA sees is its own root with
+// an error fragment, never an external bounce.
+//
+// Why we need this: oidc_config.post_login_redirect is set via the
+// admin cli. An admin-key compromise OR a misconfiguration that points
+// it at an external domain would otherwise let any /oidc/callback
+// error path redirect the user there with `?oidc_error=...` attached,
+// usable for phishing ("dialectic login failed: please re-enter your
+// password at evil.com/login").
+func (h *AuthHandler) safeRedirectBase(raw string) string {
+	if raw == "" {
+		return "/"
+	}
+	// Relative paths are always safe (same-origin by definition).
+	if strings.HasPrefix(raw, "/") && !strings.HasPrefix(raw, "//") {
+		return raw
+	}
+	u, err := url.Parse(raw)
+	if err != nil || u.Host == "" {
+		return "/"
+	}
+	// Walk the allow-list. cors.AllowedOrigins entries look like
+	// "https://dialectic.hangman-lab.top" — parse host out + compare.
+	for _, origin := range h.allowedHosts {
+		o, err := url.Parse(origin)
+		if err == nil && o.Host != "" && strings.EqualFold(o.Host, u.Host) {
+			return raw
+		}
+	}
+	return "/"
 }
 
 func (h *AuthHandler) Status(w http.ResponseWriter, r *http.Request) {
@@ -60,8 +100,8 @@ func (h *AuthHandler) Callback(w http.ResponseWriter, r *http.Request) {
 		// something useful instead of a 500 page mid-login.
 		c, _ := h.oidc.GetConfig(r.Context())
 		base := "/"
-		if c != nil && c.PostLoginRedirect != "" {
-			base = c.PostLoginRedirect
+		if c != nil {
+			base = h.safeRedirectBase(c.PostLoginRedirect)
 		}
 		sep := "#"
 		if strings.Contains(base, "#") {
@@ -70,11 +110,15 @@ func (h *AuthHandler) Callback(w http.ResponseWriter, r *http.Request) {
 		http.Redirect(w, r, base+sep+"oidc_error="+url.QueryEscape(err.Error()), http.StatusFound)
 		return
 	}
+	// Validate the success-path redirect too — HandleCallback returned
+	// PostLoginRedirect from the same DB row, so the same open-redirect
+	// risk applies on the happy path.
+	safe := h.safeRedirectBase(redirect)
 	sep := "#"
-	if strings.Contains(redirect, "#") {
+	if strings.Contains(safe, "#") {
 		sep = "&"
 	}
-	http.Redirect(w, r, redirect+sep+"oidc_ticket="+url.QueryEscape(ticket), http.StatusFound)
+	http.Redirect(w, r, safe+sep+"oidc_ticket="+url.QueryEscape(ticket), http.StatusFound)
 }
 
 type exchangeBody struct {