feat(spa-server): initial implementation — SPA/PWA container server (v1.0.0)

Introduces code.nochebuena.dev/einherjar/spa-server — a container-first HTTP
server for single-page applications and progressive web apps. Produces a Docker
base image; downstream SPA Dockerfiles require only a single COPY instruction.

Interfaces and types (CT-6: one TypeSpec per file):
- Config — Port (EINHERJAR_SPA_PORT, default 8080) and StaticDir
  (EINHERJAR_SPA_STATIC_DIR, default /srv/www); parsed via os.Getenv
- Server — implements lifecycle.Component; registered with launcher.New

Implementation:
- NewServer(logger, cfg) *Server — returns a lifecycle-managed HTTP server
- OnInit: builds http.NewServeMux with GET /health and /* routes
- OnStart: binds TCP listener synchronously via net.Listen (port conflicts
  surface immediately), then serves in a background goroutine
- OnStop: graceful shutdown via http.Server.Shutdown with 10s timeout

spa.NewHandler(logger, staticDir) http.Handler:
- Uses http.Dir.Open for path sanitisation (prevents directory traversal)
- File exists and is not a directory → served by http.FileServer
- File missing or is a directory → serves index.html (SPA router owns all routes)
- Directory listing is disabled

health.NewHandler(logger) http.Handler:
- Always responds 200 {"status":"UP","components":{}}
- Wire format matches web/health.Response without importing the web module
- No external checks — process reachability is the only meaningful health signal

Config (EINHERJAR_SPA_* env vars):
  Port(8080), StaticDir(/srv/www)

Docker:
- Multi-stage: golang:1.26-alpine builder → alpine:3.21 runtime
- Image published as:
    code.nochebuena.dev/einherjar/spa-server:v1.0.0
    code.nochebuena.dev/einherjar/spa-server:latest
- VOLUME ["/srv/www"] declared; consumer overrides via COPY or volume mount

No external dependencies beyond contracts, core, and the Go standard library.

health/handler.go

@@ -0,0 +1,35 @@
+// Package health provides the /health HTTP handler for spa-server.
+// The wire format is identical to web/health.Response so responses are
+// consistent across all Einherjar modules — without importing the web module.
+package health
+
+import (
+	"encoding/json"
+	"net/http"
+
+	"code.nochebuena.dev/einherjar/contracts/logging"
+)
+
+// response mirrors web/health.Response: {"status":"UP","components":{}}.
+type response struct {
+	Status     string            `json:"status"`
+	Components map[string]string `json:"components"`
+}
+
+type handler struct {
+	logger logging.Logger
+}
+
+// NewHandler returns an http.Handler that always responds 200 UP.
+// spa-server has no external dependencies to probe; being reachable is the health check.
+func NewHandler(logger logging.Logger) http.Handler {
+	return &handler{logger: logger}
+}
+
+func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(http.StatusOK)
+	if err := json.NewEncoder(w).Encode(response{Status: "UP", Components: map[string]string{}}); err != nil {
+		h.logger.Error("health: encode response", err)
+	}
+}