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/e2e/error-boundary.spec.ts

@@ -0,0 +1,42 @@
+import { test, expect, type Page } from './fixtures';
+
+// The root +error.svelte catches load() errors that a page rethrows rather than
+// capturing in-band (e.g. the profile overview). Without it these fell through
+// to SvelteKit's bare default error page — or blanked entirely on a raw
+// TypeError. Drive the profile loader into a failure and assert the boundary.
+
+async function authed(page: Page) {
+    await page.route('**/api/v1/auth/config', (r) =>
+        r.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({ self_register_enabled: true, private_mode: false })
+        })
+    );
+    await page.route('**/api/v1/auth/me', (r) =>
+        r.fulfill({
+            status: 200,
+            contentType: 'application/json',
+            body: JSON.stringify({
+                user: { id: 'u1', username: 'reader', created_at: '2026-01-01T00:00:00Z', is_admin: false }
+            })
+        })
+    );
+    await page.route('**/api/v1/auth/me/preferences', (r) =>
+        r.fulfill({ status: 200, contentType: 'application/json', body: '{}' })
+    );
+}
+
+test('profile: a network failure renders the error boundary, not a blank page', async ({
+    page
+}) => {
+    await authed(page);
+    // Profile's load rethrows non-401 errors; a raw abort is the TypeError case
+    // the client now normalises to an ApiError so it reaches the boundary.
+    await page.route('**/api/v1/me/bookmarks*', (r) => r.abort());
+    await page.route('**/api/v1/me/collections*', (r) => r.abort());
+
+    await page.goto('/profile');
+    await expect(page.getByTestId('error-boundary')).toBeVisible();
+    await expect(page.getByRole('button', { name: 'Try again' })).toBeVisible();
+});