Add warning about default routes

I hope this puts a lot of questions about SIM cards to rest. I found
that the warning also sometimes applies to "dead" SIM cards which have
expired a long time ago.

Run `busybox ip route` to determine whether the device has an active SIM
card. That command has been manually tested on Moxee, Orbic and TP-Link.
It's prefixed with `busybox` because that makes it more likely it would
work on UZ801, though it wasn't tested there. If the command invocation
fails, the alert is suppressed and a warning is logged.

The command is only run once on pageload. It could've been part of the
status endpoint, but then the UI would poll it way too often.

daemon/web/src/lib/components/IPRouteAlert.svelte

@@ -0,0 +1,52 @@
+<script lang="ts">
+    import { get_route_status } from '$lib/utils.svelte';
+    import WarningIcon from './WarningIcon.svelte';
+
+    let show_alert = $state(false);
+    let check_completed = $state(false);
+
+    async function check_route() {
+        if (check_completed) return;
+
+        try {
+            const status = await get_route_status();
+            if (!status.has_default_route) {
+                show_alert = true;
+            }
+        } catch (err) {
+            console.error('Failed to check route status:', err);
+        }
+        check_completed = true;
+    }
+
+    function dismiss() {
+        show_alert = false;
+    }
+
+    $effect(() => {
+        check_route();
+    });
+</script>
+
+{#if show_alert}
+    <div
+        class="bg-yellow-100 border-yellow-400 drop-shadow p-4 flex flex-col gap-2 border rounded-md"
+    >
+        <span class="text-xl font-bold flex flex-row items-center gap-2 text-yellow-700">
+            <WarningIcon class="w-6 h-6 text-yellow-600" />
+            No Default Route Detected
+        </span>
+        <p>
+            This device didn't get an IP address from the network operator. Presumably the SIM card
+            is not inserted or very old. Try a different SIM card.
+        </p>
+        <div class="flex flex-row gap-2 justify-end">
+            <button
+                class="font-medium py-2 px-4 rounded-md border border-gray-400 hover:bg-yellow-200"
+                onclick={dismiss}
+            >
+                Dismiss
+            </button>
+        </div>
+    </div>
+{/if}