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.

cmd/spa-server/main.go

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"fmt"
+	"log/slog"
+	"os"
+
+	"code.nochebuena.dev/einherjar/core/launcher"
+	"code.nochebuena.dev/einherjar/core/logz"
+	spaserver "code.nochebuena.dev/einherjar/spa-server"
+)
+
+func main() {
+	logCfg := logz.Config{JSON: true}
+	if lvl := os.Getenv("EINHERJAR_LOG_LEVEL"); lvl != "" {
+		var level slog.Level
+		if err := level.UnmarshalText([]byte(lvl)); err == nil {
+			logCfg.Level = level
+		}
+	}
+
+	logger := logz.New(logCfg)
+	cfg := spaserver.DefaultConfig()
+	srv := spaserver.NewServer(logger, cfg)
+
+	lc := launcher.New(logger)
+	lc.Append(srv)
+
+	if err := lc.Run(); err != nil {
+		fmt.Fprintln(os.Stderr, "fatal:", err)
+		os.Exit(1)
+	}
+}