fix: define silenceAlerts() — was called but never written

Restores the original behavior from before commit 46b0cb9:
delegates to logic.onTouch() which transitions state to SILENT.

This was a pre-existing bug (undefined function) exposed by the
multi-target refactoring changing compile guard structure.

sketches/doorbell-touch-esp32-32e/doorbell-touch-esp32-32e.ino

@@ -61,6 +61,12 @@ void setup() {
   Serial.println("[BOOT] Ready — monitoring ntfy.sh\n");
 }
 
+// ── Silence handler (delegates to DoorbellLogic) ────────────────
+void silenceAlerts() {
+  Serial.println("[SILENCE] User completed hold-to-silence gesture");
+  logic.onTouch(TouchEvent{true, 0, 0});
+}
+
 void loop() {
   logic.update();