feat(router): improve error handling with dedicated error pages

- Add renderErrorPage helper for consistent error rendering
- Add ensureUserExistsMiddleware to detect deleted users and redirect to login
- Add catch-all 404 handler for unknown routes
- Gracefully handle data loading failures with error messages instead of crashing
- Log errors for debugging while still rendering pages

internal/router/helpers.go

@@ -2,6 +2,7 @@ package router
 
 import (
 	"context"
+	"log"
 
 	"bookhoard/templates"
 
@@ -19,11 +20,13 @@ func getTemplateUserWithTheme(c echo.Context, cfg *Config) (templates.User, erro
 
 	userUUID, err := uuid.Parse(userID)
 	if err != nil {
+		log.Printf("getTemplateUserWithTheme failed: invalid UUID '%s': %v", userID, err)
 		return templates.User{}, err
 	}
 
 	userDB, err := cfg.Queries.GetUser(c.Request().Context(), uuidToPGType(userUUID))
 	if err != nil {
+		log.Printf("getTemplateUserWithTheme failed: database query error for user ID %s: %v", userID, err)
 		return templates.User{}, err
 	}