feat: WiFiManager captive portal provisioning

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

firmware/src/provisioning.cpp

@@ -0,0 +1,24 @@
+// firmware/src/provisioning.cpp
+#include "provisioning.h"
+#include "config.h"
+#include <WiFiManager.h>
+
+bool provisioning_run(uint32_t timeout_ms) {
+    WiFiManager wm;
+    wm.setConfigPortalTimeout(timeout_ms / 1000);
+    wm.setTitle("DoorCounter Setup");
+    wm.setCustomHeadElement(
+        "<style>"
+        "body{font-family:sans-serif;max-width:400px;margin:40px auto;padding:0 16px}"
+        "h1{font-size:1.2em;color:#333}"
+        "p{color:#666;font-size:.9em}"
+        "</style>"
+    );
+
+    bool connected = wm.startConfigPortal("DoorCounter-Setup");
+
+    if (connected) {
+        config_save_wifi(wm.getWiFiSSID(), wm.getWiFiPass());
+    }
+    return connected;
+}