Stage 17 round 6 (#16-20): landing page, /app/ move, cache + stream fixes

Close out Stage 17 round 6:

- Landing page at / — one Vite build with two entries (index.html = game
  SPA, landing.html = a lightweight landing reusing the theme/i18n/
  aboutContent leaf modules, not the app store).
- Move the web game SPA to /app/; the Telegram Mini App stays at /telegram/
  (gateway webui.Handler(stripPrefix, indexName): landing at /, SPA at /app/
  + /telegram/). Per-language "Play in Telegram" link via new
  VITE_TELEGRAM_LINK_EN/_RU build vars (button hides when unset).
- Cache headers: hash-named /assets/* immutable, HTML shells no-cache (the
  go:embed zero modtime emitted no validators, so the client re-downloaded
  the whole bundle every launch).
- Live-stream 15s abort fix: an immediate heartbeat on open + a 10s default
  interval (the first tick at 15s raced the edge idle timeout -> reconnect
  storm).

PLAN/ARCHITECTURE(§13)/FUNCTIONAL(+ru)/gateway+ui+deploy READMEs updated;
round 6 closed. Tests: gateway webui/connectsrv units, ui landing unit + e2e,
full e2e (60) green.

gateway/internal/webui/webui_test.go

@@ -16,37 +16,50 @@ func get(t *testing.T, h http.Handler, target string) *http.Response {
 	return rec.Result()
 }
 
-func TestHandlerServesIndexAndFallsBack(t *testing.T) {
-	h := Handler("")
+func body(t *testing.T, resp *http.Response) string {
+	t.Helper()
+	b, _ := io.ReadAll(resp.Body)
+	return string(b)
+}
 
-	// The embedded placeholder index is served at the root.
-	if resp := get(t, h, "/"); resp.StatusCode != http.StatusOK {
-		t.Fatalf("GET / status = %d, want 200", resp.StatusCode)
-	}
+// TestLandingMountServesLandingAndFallsBack: "/" serves the landing shell (no-cache) and
+// any unknown path falls back to it.
+func TestLandingMountServesLandingAndFallsBack(t *testing.T) {
+	h := Handler("", "landing.html")
 
-	// An existing (non-index) file is served directly by the file server.
-	if resp := get(t, h, "/assets/.gitkeep"); resp.StatusCode != http.StatusOK {
-		t.Fatalf("GET /assets/.gitkeep status = %d, want 200 (served file)", resp.StatusCode)
+	resp := get(t, h, "/")
+	if resp.StatusCode != http.StatusOK || !strings.Contains(body(t, resp), "scrabble-landing") {
+		t.Fatalf("GET / did not serve the landing shell (status %d)", resp.StatusCode)
 	}
-
-	// An unknown deep link falls back to the SPA shell (200, not 404) so the
-	// client-side hash router can take over.
-	resp := get(t, h, "/game/abc/deep")
-	if resp.StatusCode != http.StatusOK {
-		t.Fatalf("GET /game/abc/deep status = %d, want 200 (SPA fallback)", resp.StatusCode)
+	if cc := get(t, h, "/").Header.Get("Cache-Control"); cc != "no-cache" {
+		t.Errorf("landing Cache-Control = %q, want no-cache", cc)
 	}
-	body, _ := io.ReadAll(resp.Body)
-	if !strings.Contains(string(body), "<html") {
-		t.Fatalf("fallback body is not the index HTML: %q", body)
+	if resp := get(t, h, "/whatever"); resp.StatusCode != http.StatusOK {
+		t.Fatalf("GET /whatever status = %d, want 200 (fallback)", resp.StatusCode)
 	}
 }
 
-func TestHandlerStripsPrefix(t *testing.T) {
-	h := Handler("/telegram/")
+// TestAppMountServesShellStripsPrefixAndCachesAssets: "/app/" and "/telegram/" serve the app
+// shell (index.html), strip their prefix, fall back for deep links, and mark hash-named
+// assets immutable.
+func TestAppMountServesShellStripsPrefixAndCachesAssets(t *testing.T) {
+	for _, prefix := range []string{"/app/", "/telegram/"} {
+		h := Handler(prefix, "index.html")
 
-	for _, target := range []string{"/telegram/", "/telegram/assets/.gitkeep", "/telegram/lobby/x"} {
-		if resp := get(t, h, target); resp.StatusCode != http.StatusOK {
-			t.Fatalf("GET %s status = %d, want 200", target, resp.StatusCode)
+		if resp := get(t, h, prefix); resp.StatusCode != http.StatusOK || !strings.Contains(body(t, resp), "scrabble-app-shell") {
+			t.Fatalf("GET %s did not serve the app shell (status %d)", prefix, resp.StatusCode)
+		}
+		// A deep link falls back to the shell so the hash router can take over.
+		if resp := get(t, h, prefix+"game/abc"); resp.StatusCode != http.StatusOK || !strings.Contains(body(t, resp), "scrabble-app-shell") {
+			t.Fatalf("GET %sgame/abc did not fall back to the app shell (status %d)", prefix, resp.StatusCode)
+		}
+		// A hash-named asset is served directly and marked immutable.
+		resp := get(t, h, prefix+"assets/.gitkeep")
+		if resp.StatusCode != http.StatusOK {
+			t.Fatalf("GET %sassets/.gitkeep status = %d, want 200", prefix, resp.StatusCode)
+		}
+		if cc := resp.Header.Get("Cache-Control"); !strings.Contains(cc, "immutable") {
+			t.Errorf("asset Cache-Control = %q, want immutable", cc)
 		}
 	}
 }