Phase 3: embed web UI with go:embed, pinned TypeScript toolchain, drop i18n, add /healthz

- html/embed.go embeds html/ (pages, css, img, js, video); src/assets.go
  serves it, with os.DirFS("html") under -dev. Static assets get an ETag
  and Cache-Control: no-cache; HTML pages are still templated (only the
  login error message is substituted now).
- Delete the generated src/webUI.go (783 KB base64), src/html-build.go and
  cmd/webui-gen; Dockerfile no longer runs a generator.
- Language layer removed: 254 {{.x}} placeholders inlined as English
  strings in ts/*.ts and the two auth pages; html/lang/en.json, the
  LanguageUI struct and the 'language' setting are gone.
- ts/tsconfig.json + package.json pin typescript 5.9.3; the seven sources
  compile (ES2020, global scripts) into one committed html/js/app.js.
  Ten unreferenced legacy scripts under html/js/ deleted; all pages load
  js/app.js.
- Fix the six type errors that blocked a clean compile, including a real
  bug: a missing semicolon in the search shortcut handler made the code
  call the result of preventDefault(), so the shortcut threw instead of
  focusing the search box.
- /healthz liveness endpoint; Dockerfile healthcheck and README use it.
- Drone: go vet, and a webui-check step that rebuilds the bundle and fails
  if the committed app.js is stale.
- README-DEV.md documents build, UI toolchain, -dev, versioning, CI.

src/assets.go

@@ -0,0 +1,52 @@
+package src
+
+import (
+	"crypto/sha256"
+	"encoding/hex"
+	"io/fs"
+	"os"
+	"path"
+	"strings"
+	"sync"
+
+	webui "xteve/html"
+)
+
+// webFS serves the web interface. It is the embedded copy by default and the
+// local html/ directory when xTeVe runs with -dev.
+var webFS fs.FS = webui.FS
+
+var webETags sync.Map // path -> ETag, only used when not in dev mode
+
+// initWebFS : selects embedded or on-disk web files.
+func initWebFS(dev bool) {
+	if dev {
+		webFS = os.DirFS("html")
+		return
+	}
+	webFS = webui.FS
+}
+
+// readWebFile : reads a file from the web interface, name relative to html/.
+func readWebFile(name string) ([]byte, error) {
+	name = strings.TrimPrefix(path.Clean("/"+name), "/")
+	if name == "" || name == "." {
+		return nil, fs.ErrNotExist
+	}
+	return fs.ReadFile(webFS, name)
+}
+
+// webFileETag : content hash used for HTTP caching of static web assets.
+func webFileETag(name string, content []byte) string {
+	if !System.Dev {
+		if v, ok := webETags.Load(name); ok {
+			return v.(string)
+		}
+	}
+	sum := sha256.Sum256(content)
+	etag := `"` + hex.EncodeToString(sum[:16]) + `"`
+	if !System.Dev {
+		webETags.Store(name, etag)
+	}
+	return etag
+}