Harden boot path + surface errors instead of blanking

A friend on Chrome reported the page renders briefly then drops to the
plain theme background (Brave/Chromium fine). Defensive changes so the
next blank-page report comes with a visible cause:

- main.ts installs Vue config.errorHandler, window.onerror, and
  window.unhandledrejection listeners. Any uncaught error pins a red
  banner to the top of the page with the error message, so the user
  sees what crashed instead of a blank background.
- App.vue's onMounted now wraps each data fetch in a safeLoad helper.
  Promise.allSettled (vs allSettled used previously: was a plain
  Promise.all) means one failed file can't reject the whole batch.
  Errors log to the console and show as a non-fatal "Boot warning"
  toast in the share bar.

This doesn't fix the root cause yet — needs Chrome devtools console
output to pinpoint — but ensures the next person who hits it sees an
actionable error.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

character-builder/frontend/src/App.vue

@@ -65,27 +65,46 @@ watch(
   { immediate: true },
 );
 
+const bootError = ref<string>('');
+
+async function safeLoad<T>(label: string, fn: () => Promise<T>): Promise<T | null> {
+  try {
+    return await fn();
+  } catch (e: any) {
+    const msg = `[${label}] ${e?.message || e}`;
+    // eslint-disable-next-line no-console
+    console.error(msg, e);
+    if (!bootError.value) bootError.value = msg;
+    return null;
+  }
+}
+
 onMounted(async () => {
-  await Promise.all([
+  // Each load is isolated — a single failed fetch can't bring down the rest
-    loadCharacterXp().then((d) => (charXp.value = d)),
+  // of the page (or leave us looking at a blank background).
+  await Promise.allSettled([
+    safeLoad('character-xp', loadCharacterXp).then(
+      (d) => d && (charXp.value = d),
+    ),
     ...SPECS.map((s) =>
-      loadSpec(s).then((d) => (specTables.value = { ...specTables.value, [s]: d })),
+      safeLoad(`spec-${s}`, () => loadSpec(s)).then(
+        (d) => d && (specTables.value = { ...specTables.value, [s]: d }),
+      ),
     ),
-    loadFaction('atreides').then(
+    safeLoad('faction-atreides', () => loadFaction('atreides')).then(
-      (d) => (factionTables.value = { ...factionTables.value, atreides: d }),
+      (d) => d && (factionTables.value = { ...factionTables.value, atreides: d }),
     ),
-    loadFaction('harkonnen').then(
+    safeLoad('faction-harkonnen', () => loadFaction('harkonnen')).then(
-      (d) => (factionTables.value = { ...factionTables.value, harkonnen: d }),
+      (d) => d && (factionTables.value = { ...factionTables.value, harkonnen: d }),
     ),
   ]);
-  // Load all skill trees in background so class switching is instant
+
   for (const c of CLASSES) {
-    loadSkillTree(c.id).then(
+    safeLoad(`skills-${c.id}`, () => loadSkillTree(c.id)).then(
-      (d) => (skillTrees.value = { ...skillTrees.value, [c.id]: d }),
+      (d) => d && (skillTrees.value = { ...skillTrees.value, [c.id]: d }),
     );
   }
 
-  // If URL has ?b=<code>, load that build.
   const params = new URLSearchParams(window.location.search);
   const code = params.get('b');
   if (code) {
@@ -236,6 +255,12 @@ const specMeta: Record<SpecId, { name: string; sym: string }> = {
       <div class="grow"></div>
       <span v-if="shareLink" class="share-link">{{ shareLink }}</span>
       <span v-if="shareStatus" class="toast">{{ shareStatus }}</span>
+      <span v-if="bootError" style="
+        color: var(--ember);
+        font-family: 'JetBrains Mono', monospace;
+        font-size: 11px;
+        letter-spacing: 0.1em;
+      ">Boot warning: {{ bootError }}</span>
       <button @click="newBuild">New</button>
       <button class="primary" @click="share" :disabled="saving">
         {{ saving ? 'Saving…' : 'Share Build' }}

character-builder/frontend/src/main.ts

@@ -2,4 +2,53 @@ import { createApp } from 'vue';
 import App from './App.vue';
 import './styles.css';
 
-createApp(App).mount('#app');
+const app = createApp(App);
+
+// Surface unhandled component errors visibly. The default is to swallow
+// them, which on some browsers (we've seen this on certain Chrome builds)
+// can leave the user staring at a blank background.
+app.config.errorHandler = (err, _instance, info) => {
+  // eslint-disable-next-line no-console
+  console.error('[vue] component error:', err, info);
+  showBootBanner(`Vue error: ${(err as Error)?.message || err}`);
+};
+
+// Unhandled promise rejections fall through Vue's handler; catch them
+// at the window level so the user can see them.
+window.addEventListener('unhandledrejection', (e) => {
+  const msg = (e.reason && (e.reason.message || e.reason.toString())) || 'unknown';
+  // eslint-disable-next-line no-console
+  console.error('[window] unhandledrejection:', e.reason);
+  showBootBanner(`Unhandled error: ${msg}`);
+});
+
+window.addEventListener('error', (e) => {
+  // eslint-disable-next-line no-console
+  console.error('[window] error:', e.error || e.message);
+  showBootBanner(`Script error: ${e.message || 'unknown'}`);
+});
+
+function showBootBanner(text: string) {
+  if (document.getElementById('boot-banner')) return; // only show the first
+  const el = document.createElement('div');
+  el.id = 'boot-banner';
+  el.textContent = text;
+  el.style.cssText = [
+    'position:fixed',
+    'top:0',
+    'left:0',
+    'right:0',
+    'z-index:99999',
+    'background:#3a1a14',
+    'color:#f4e9d8',
+    'border-bottom:1px solid #c8643a',
+    'padding:10px 16px',
+    'font-family:monospace',
+    'font-size:12px',
+    'white-space:pre-wrap',
+    'word-break:break-word',
+  ].join(';');
+  document.body.appendChild(el);
+}
+
+app.mount('#app');