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.

config.go

@@ -0,0 +1,36 @@
+package spaserver
+
+import (
+	"os"
+	"strconv"
+)
+
+// Config holds the server configuration.
+// All fields are populated from environment variables by [DefaultConfig].
+type Config struct {
+	// Port is the TCP port the server listens on.
+	Port int `env:"EINHERJAR_SPA_PORT" envDefault:"8080"`
+	// StaticDir is the filesystem path that contains index.html and all assets.
+	StaticDir string `env:"EINHERJAR_SPA_STATIC_DIR" envDefault:"/srv/www"`
+}
+
+// DefaultConfig returns a Config populated from environment variables,
+// falling back to safe defaults when variables are unset.
+func DefaultConfig() Config {
+	port := 8080
+	if v := os.Getenv("EINHERJAR_SPA_PORT"); v != "" {
+		if p, err := strconv.Atoi(v); err == nil && p > 0 {
+			port = p
+		}
+	}
+
+	staticDir := "/srv/www"
+	if v := os.Getenv("EINHERJAR_SPA_STATIC_DIR"); v != "" {
+		staticDir = v
+	}
+
+	return Config{
+		Port:      port,
+		StaticDir: staticDir,
+	}
+}