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/package.json

@@ -1,6 +1,6 @@
 {
   "name": "mangalord-frontend",
-  "version": "0.125.1",
+  "version": "0.125.2",
   "private": true,
   "type": "module",
   "scripts": {