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/lib/api/client.ts

@@ -103,7 +103,21 @@ export async function request<T>(
     // working. For same-origin requests this is a no-op compared to the
     // default 'same-origin', so the same-origin happy path is
     // unchanged.
-    const res = await fetch(`${BASE}${path}`, { credentials: 'include', ...init });
+    let res: Response;
+    try {
+        res = await fetch(`${BASE}${path}`, { credentials: 'include', ...init });
+    } catch {
+        // A network-level failure (connection refused, DNS, offline, blocked by
+        // CORS) rejects `fetch` with a bare TypeError instead of returning a
+        // response. Normalise it to an ApiError (status 0 = "no response reached
+        // us") so callers and SvelteKit load functions handle it uniformly
+        // instead of crashing on an unexpected TypeError.
+        throw new ApiError(
+            0,
+            'network_error',
+            'Could not reach the server. Check your connection and try again.'
+        );
+    }
     if (!res.ok) {
         let code = 'http_error';
         let message = `${res.status} ${res.statusText}`;