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>

tests/provisioning/test_portal.py

@@ -0,0 +1,47 @@
+import pytest
+
+from planemapper.provisioning.portal import app
+
+
+@pytest.fixture
+def client():
+    app.config["TESTING"] = True
+    with app.test_client() as c:
+        yield c
+
+
+def test_index_returns_200(client) -> None:
+    resp = client.get("/")
+    assert resp.status_code == 200
+
+
+def test_index_contains_form_fields(client) -> None:
+    resp = client.get("/")
+    data = resp.data.decode()
+    assert 'name="location"' in data
+    assert 'name="radius"' in data
+    assert 'name="wifi_ssid"' in data
+    assert 'name="wifi_password"' in data
+    assert "Find location" in data
+    assert "Set up device" in data
+
+
+def test_generate_204_redirects_to_index(client) -> None:
+    resp = client.get("/generate_204")
+    assert resp.status_code in (301, 302)
+    assert resp.headers["Location"].endswith("/")
+
+
+def test_hotspot_detect_redirects_to_index(client) -> None:
+    resp = client.get("/hotspot-detect.html")
+    assert resp.status_code in (301, 302)
+
+
+def test_ncsi_redirects_to_index(client) -> None:
+    resp = client.get("/ncsi.txt")
+    assert resp.status_code in (301, 302)
+
+
+def test_unknown_route_redirects_to_index(client) -> None:
+    resp = client.get("/some/random/path")
+    assert resp.status_code in (301, 302)