feat(gateway,ui): client-version gate — turn away too-old builds

Introduce a minimum-supported-client gate so a future incompatible wire change
can turn away installed builds too old to speak it, cleanly, instead of letting
them crash on decode. It rides the outermost stable layer (an HTTP header), never
the FlatBuffers payload.

Gateway:
- New internal/clientver: dependency-free parse + compare of the leading
  MAJOR.MINOR.PATCH (a git-describe suffix is tolerated).
- GATEWAY_MIN_CLIENT_VERSION config (empty => gate dormant; validated at load).
- connectsrv checks the X-Client-Version header before decoding the payload:
  Execute returns result_code="update_required" (before the registry lookup),
  Subscribe returns FailedPrecondition. It fails open on an absent or garbled
  header — the header is a client-controlled compatibility signal, not an access
  control.

Client:
- Attach X-Client-Version on every call.
- A terminal update.svelte.ts store + a non-dismissable UpdateOverlay (native
  opens VITE_STORE_URL, web reloads); retry.ts maps FailedPrecondition to the
  update_required sentinel; a mock __update hook drives the e2e.

Wire-additive and contour-safe: no FBS/proto regen, no schema migration; the gate
stays dormant until GATEWAY_MIN_CLIENT_VERSION is deliberately set, so web / VK /
Telegram behaviour is unchanged. The silent reconciliation seam is deferred to the
offline-first work (its only caller). Tests: Go clientver/config/connectsrv gate
tests, retry.test.ts, Playwright update.spec.ts.

gateway/internal/config/config_test.go

@@ -47,6 +47,29 @@ func TestLoadMaxBodyBytes(t *testing.T) {
 	}
 }
 
+// TestLoadMinClientVersion verifies the client-version gate config: dormant (empty) by
+// default, a parseable version accepted, and an unparseable one rejected.
+func TestLoadMinClientVersion(t *testing.T) {
+	c, err := Load()
+	if err != nil {
+		t.Fatalf("Load: %v", err)
+	}
+	if c.MinClientVersion != "" {
+		t.Errorf("MinClientVersion = %q, want empty (gate dormant)", c.MinClientVersion)
+	}
+	t.Setenv("GATEWAY_MIN_CLIENT_VERSION", "v1.16.0")
+	if c, err = Load(); err != nil {
+		t.Fatalf("Load with a valid min version: %v", err)
+	}
+	if c.MinClientVersion != "v1.16.0" {
+		t.Errorf("MinClientVersion = %q, want %q", c.MinClientVersion, "v1.16.0")
+	}
+	t.Setenv("GATEWAY_MIN_CLIENT_VERSION", "dev")
+	if _, err := Load(); err == nil {
+		t.Fatal("Load: expected an error for an unparseable GATEWAY_MIN_CLIENT_VERSION, got nil")
+	}
+}
+
 // TestLoadAbuseDefaults verifies the anti-abuse ban defaults: disabled (prod-only),
 // the agreed thresholds, and no honeytoken.
 func TestLoadAbuseDefaults(t *testing.T) {