R3: split the landing into its own static container

- gateway/Dockerfile gains a `landing` target: caddy:2-alpine + the shared
  Vite build (identical build args keep the ui stage a single cached build);
  the gateway target drops landing.html from the embed.
- The contour caddy routes /app/, /telegram/ and the Connect path to the
  gateway; the catch-all — the landing at / and any stray path — goes to the
  new landing service, so junk traffic is absorbed by static file serving.
- deploy/landing/Caddyfile mirrors the webui caching (immutable assets,
  no-cache shells) and falls back unknown paths to the landing shell.
- The gateway's / now 308-redirects to /app/ (keeps a local no-caddy run
  usable); webui placeholder landing.html removed.
- CI deploy probe checks both / (landing) and /app/ (gateway).

Verified: both images build; the landing container serves landing.html at /
(no-cache) with junk-path fallback; the gateway image redirects / to /app/
and carries no landing content.

gateway/internal/connectsrv/server_test.go

@@ -197,6 +197,26 @@ func TestExecuteOversizedPayloadRejected(t *testing.T) {
 	}
 }
 
+// TestRootRedirectsToApp verifies the gateway no longer serves a landing at "/"
+// (it lives in the landing container since R3): a stray root hit is redirected
+// to the app shell.
+func TestRootRedirectsToApp(t *testing.T) {
+	front := httptest.NewServer(connectsrv.NewServer(connectsrv.Deps{}).HTTPHandler())
+	defer front.Close()
+
+	client := &http.Client{CheckRedirect: func(*http.Request, []*http.Request) error {
+		return http.ErrUseLastResponse
+	}}
+	resp, err := client.Get(front.URL + "/")
+	if err != nil {
+		t.Fatalf("get /: %v", err)
+	}
+	defer func() { _ = resp.Body.Close() }()
+	if resp.StatusCode != http.StatusPermanentRedirect || resp.Header.Get("Location") != "/app/" {
+		t.Fatalf("GET / = %d -> %q, want 308 -> /app/", resp.StatusCode, resp.Header.Get("Location"))
+	}
+}
+
 func TestExecuteUnknownMessageType(t *testing.T) {
 	client, cleanup := newEdge(t, func(w http.ResponseWriter, r *http.Request) {})
 	defer cleanup()