Implement story 1.3: WiFi hotspot and captive portal form

Implement wifi.py (start_ap/stop_ap/kill_wifi with subprocess and ProvisioningError),
portal.py (Flask captive portal with Android/iOS/Windows probe redirects and 404 catch-all),
updated provision.py provisioning loop, and full test suite (38 passing).

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

src/planemapper/provision.py

@@ -1,8 +1,28 @@
 import logging
 
+from planemapper.provisioning import ProvisioningError, wifi
+
 log = logging.getLogger(__name__)
 
 
+def _reset_to_portal_state() -> None:
+    log.info("Resetting to portal state")
+    try:
+        wifi.stop_ap()
+    except Exception:
+        pass
+
+
 def main() -> None:
     logging.basicConfig(level=logging.INFO)
-    log.info("not implemented")
+    provisioned = False
+    while not provisioned:
+        try:
+            wifi.start_ap()
+            # Portal runs here (Story 1.3+ wires Flask app)
+            # Provisioning sequence continues in Story 1.5
+            log.info("Provisioning sequence started")
+            provisioned = True  # placeholder — full sequence wired in 1.5
+        except ProvisioningError as e:
+            log.error("Provisioning failed: %s", e)
+            _reset_to_portal_state()