feat: root error boundary and network-error normalisation

A load() that rethrew (e.g. the profile overview) fell through to SvelteKit's
bare default error page, and a connection-refused fetch rejected as a raw
TypeError that blanked the view. Wrap fetch network failures in the client as
ApiError(status 0, network_error) so callers handle them uniformly, and add a
root +error.svelte with a friendly message plus Try again / Go home actions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/src/routes/+error.svelte

@@ -0,0 +1,73 @@
+<script lang="ts">
+    import { page } from '$app/stores';
+    import { goto, invalidateAll } from '$app/navigation';
+
+    // Shown whenever a load function throws (including a wrapped network_error
+    // ApiError from the client). Without this boundary an uncaught load error
+    // fell through to SvelteKit's bare default error page — or, for a raw
+    // TypeError, blanked the view entirely.
+    let status = $derived($page.status);
+    let message = $derived($page.error?.message || 'Something went wrong.');
+    let isNetwork = $derived(status === 0 || status >= 500);
+
+    async function retry() {
+        // Re-run the failed load(s) in place rather than a hard reload.
+        await invalidateAll();
+    }
+</script>
+
+<section class="error-page" data-testid="error-boundary">
+    <p class="status">{status || 'Error'}</p>
+    <h1>{isNetwork ? 'We hit a snag' : 'This page is unavailable'}</h1>
+    <p class="message">{message}</p>
+    <div class="actions">
+        <button type="button" class="primary" onclick={retry}>Try again</button>
+        <button type="button" class="ghost" onclick={() => goto('/')}>Go home</button>
+    </div>
+</section>
+
+<style>
+    .error-page {
+        max-width: 32rem;
+        margin: 4rem auto;
+        padding: 0 1.5rem;
+        text-align: center;
+    }
+    .status {
+        font-size: 2.5rem;
+        font-weight: 700;
+        color: var(--color-text-muted, #888);
+        margin: 0;
+        line-height: 1;
+    }
+    h1 {
+        margin: 0.75rem 0 0.5rem;
+        font-size: 1.4rem;
+    }
+    .message {
+        color: var(--color-text-muted, #888);
+        margin: 0 0 1.5rem;
+        overflow-wrap: anywhere;
+    }
+    .actions {
+        display: flex;
+        gap: 0.75rem;
+        justify-content: center;
+    }
+    button {
+        padding: 0.55rem 1.1rem;
+        border-radius: var(--radius-md, 8px);
+        font: inherit;
+        cursor: pointer;
+        border: 1px solid var(--color-border, #ccc);
+    }
+    .primary {
+        background: var(--color-accent, #3b82f6);
+        color: var(--color-accent-contrast, #fff);
+        border-color: transparent;
+    }
+    .ghost {
+        background: transparent;
+        color: inherit;
+    }
+</style>