Add configuration files and update project documentation

- Introduced .editorconfig for consistent coding styles across the project.
- Added .golangci.yml for Go linting configuration.
- Updated AGENTS.md to clarify project structure and components.
- Enhanced CONTRIBUTING.md with Makefile usage for common tasks.
- Updated Dockerfiles to use Go 1.24 and improved build instructions.
- Refined README.md and deployment documentation for clarity.
- Added testing documentation in testing.md for backend and frontend tests.
- Introduced Makefile for streamlined development commands and tasks.

internal/app/apperr/errors.go

@@ -0,0 +1,18 @@
+package apperr
+
+import "errors"
+
+// Domain errors returned by services. Handlers map these to HTTP status codes.
+var (
+	ErrNotFound             = errors.New("not found")
+	ErrUnauthorized         = errors.New("unauthorized")
+	ErrForbidden            = errors.New("forbidden")
+	ErrBadRequest           = errors.New("bad request")
+	ErrInternal             = errors.New("internal error")
+	ErrOAuthOnly            = errors.New("use OAuth to sign in")
+	ErrProviderUnconfigured = errors.New("OAuth provider not configured")
+	ErrStateExpired         = errors.New("OAuth state expired")
+	ErrStateMismatch        = errors.New("OAuth state mismatch")
+	ErrExchangeFailed       = errors.New("OAuth exchange failed")
+	ErrUserInfoFailed       = errors.New("failed to get user info")
+)