auth: split dev-mode auth constructor and wire up dev-login/logout UI

Squeamish about New()'s empty-domain-string sentinel for "dev mode, skip
OIDC discovery" - split into New (always makes a real OIDC discovery
call, all params required) and NewDev (no ctx/domain/credentials at all,
since none are used). main.go now branches on cfg.DevAuthEnabled to pick
the right constructor instead of main.go/config.go coordinating on when
it's safe to pass empty strings.

Also finishes out the dev-auth flow this enables: config.Load reads a
DEV_AUTH_ENABLED-aware env file and only requires Auth0 vars when dev
auth is off; a PORT config var replaces the hardcoded :8082; and the nav
UI (layout/index templates, ui router) points login/logout links at
/api/auth/dev-login and a new /api/auth/dev-logout route when dev auth
is enabled, so the whole login/logout loop works locally without a real
Auth0 app.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/api/auth/router.go

@@ -35,6 +35,7 @@ func Routes(
 	if devAuthEnabled {
 		logger.Warn("DEV_AUTH_ENABLED is set: /api/auth/dev-login is live and lets any caller authenticate as any user_id with no credentials. Never enable this outside local development.")
 		r.GET("/dev-login", response.Handler(ls.devLoginPage))
+		r.GET("/dev-logout", response.Handler(ls.devLogoutPage))
 	}
 }
 
@@ -135,3 +136,21 @@ func (s *loginSubrouter) logoutPage(c *gin.Context) (response.Response, error) {
 	return response.TemporaryRedirect(s.auth.GetLogoutURL(host).String()).
 		Cookie(cookies.Expired("access_token")), nil
 }
+
+func (s *loginSubrouter) devLogoutPage(c *gin.Context) (response.Response, error) {
+	r := c.Request
+
+	host := r.Header.Get("X-Forwarded-Host")
+	if host == "" {
+		host = r.Host
+	}
+
+	if ck, err := r.Cookie("access_token"); err == nil && ck != nil {
+		if err := s.auth.DeleteOAuthTokens(r.Context(), ck.Value); err != nil {
+			s.log.Error("failed to delete auth token", "error", err)
+		}
+	}
+
+	return response.TemporaryRedirect("/ui").
+		Cookie(cookies.Expired("access_token")), nil
+}