Fix user default surface by reading JWT from cookie

The GET / route has no auth middleware, so user_id was always
empty. Add UserIDFromCookie helper that opportunistically reads
the sb_token cookie and extracts user_id without requiring auth.
Now user preference correctly overrides the global default.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

server/pages/pages_surfaces.go

@@ -6,6 +6,8 @@ import (
 	"net/http"
 
 	"github.com/gin-gonic/gin"
+
+	"switchboard-core/middleware"
 )
 
 // SeedSurfaces writes core surface manifests to the registry table.
@@ -92,9 +94,13 @@ func (e *Engine) EnabledSurfaceIDs() []string {
 // DefaultSurfaceRedirect returns a handler for GET / that redirects to the
 // configured default surface, falling back to the first enabled extension
 // surface, then /welcome.
+//
+// This route has no auth middleware, so we opportunistically read the JWT
+// from the cookie to check user preferences. If the cookie is missing or
+// invalid, user preference is skipped (falls through to global default).
 func (e *Engine) DefaultSurfaceRedirect() gin.HandlerFunc {
 	return func(c *gin.Context) {
-		userID := c.GetString("user_id")
+		userID := middleware.UserIDFromCookie(c, e.cfg.JWTSecret)
 		target := e.resolveDefaultSurface(c.Request.Context(), userID)
 		c.Redirect(http.StatusTemporaryRedirect, e.cfg.BasePath+target)
 	}