Show the offline screen when the network link is gone; plain panel

With no address on any interface (cable out, no wifi) the kiosk now
switches to the offline screen right away and returns when the network
and server are back. An unreachable server with the network up still
leaves the page running for InfoFlow's offline cache. The panel on the
local pages loses its blue top edge.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tests/test_agent.py

@@ -100,7 +100,7 @@ def test_chromium_error_page_is_replaced(env):
     assert "offline.html" in browser.url
 
 
-def test_running_page_is_left_alone_when_network_drops(env):
+def test_running_page_is_left_alone_when_the_server_is_unreachable(env):
     """Pages with their own offline cache (e.g. InfoFlow displays) must keep running."""
     agent, browser, probe, clock, _ = env
     browser.url = "https://schermen-ijsselflat.voxip.nl"
@@ -178,3 +178,27 @@ def test_offline_page_never_receives_the_token(env, paths):
     agent.tick()
     assert "offline.html" in browser.url
     assert "geheim" not in browser.url and "3456" in browser.url
+
+
+def test_offline_page_when_the_network_link_is_gone_then_back(env):
+    agent, browser, probe, clock, _ = env
+    network = {"eth0": ["192.168.1.20/24"]}
+    agent.addresses = lambda: dict(network)
+    browser.url = "https://schermen-ijsselflat.voxip.nl"
+    agent.tick()
+    assert browser.navigations == []
+
+    network.clear()  # cable out
+    probe.ok = False
+    clock.t += 5
+    agent.tick()
+    assert "offline.html" in browser.url and "netwerkkabel" in browser.url
+    clock.t += 5
+    agent.tick()
+    assert len(browser.navigations) == 1  # no re-navigation while it stays down
+
+    network["eth0"] = ["192.168.1.20/24"]  # cable back in
+    probe.ok = True
+    clock.t += 11
+    agent.tick()
+    assert browser.url == "https://schermen-ijsselflat.voxip.nl"