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.

doc.go

@@ -0,0 +1,42 @@
+// Package spaserver provides a lifecycle-managed HTTP server for serving
+// single-page applications (SPA) and progressive web apps (PWA) from a
+// container.
+//
+// # Overview
+//
+// The server delivers static assets from a configurable directory and falls
+// back to index.html for any request path that does not map to an existing
+// file — the standard SPA routing contract. A health endpoint is included
+// at /health with the same JSON wire format used across all Einherjar modules.
+//
+// # Lifecycle
+//
+// [NewServer] returns a [lifecycle.Component] that integrates directly with
+// [launcher.New]. [Server.OnInit] builds the HTTP mux. [Server.OnStart]
+// begins serving in a goroutine. [Server.OnStop] performs a graceful shutdown.
+//
+//	cfg := spaserver.DefaultConfig()
+//	srv := spaserver.NewServer(logger, cfg)
+//
+//	lc := launcher.New(logger)
+//	lc.Append(srv)
+//	if err := lc.Run(); err != nil {
+//	    log.Fatal(err)
+//	}
+//
+// # Configuration
+//
+// All fields are read from environment variables:
+//
+//   - EINHERJAR_SPA_PORT       — listen port (default: 8080)
+//   - EINHERJAR_SPA_STATIC_DIR — path to the directory containing index.html and assets (default: /srv/www)
+//   - EINHERJAR_LOG_LEVEL      — log level: DEBUG, INFO, WARN, ERROR (default: INFO)
+//
+// # Container usage
+//
+// The module ships as a ready-to-use base image. A downstream SPA Dockerfile
+// only needs to copy the built assets:
+//
+//	FROM code.nochebuena.dev/einherjar/spa-server:v1.0.0
+//	COPY dist/ /srv/www/
+package spaserver