chore(frontend): add diagnostics before app split

- main.js: global error handlers (error + unhandledrejection), try/catch around App mount with inline fallback UI
- bindings_logging.go: FrontendLog() and LogStartupStep() backend bindings for persistent application logging
- App.js: FrontendLog() and LogStartupStep() Wails JS bindings
- docs/frontend-architecture.md: component architecture, state ownership rules, communication patterns, logging guide, troubleshooting

All existing code untouched. This is a pure additive step that enables
blank-window debugging before any App.svelte extraction begins.

frontend/src/main.js

@@ -1,5 +1,48 @@
 import App from './App.svelte'
 
-new App({
-  target: document.getElementById('app')
+// ===== Global frontend error diagnostics =====
+// These catch runtime errors that would otherwise cause a silent blank window.
+
+window.addEventListener('error', (event) => {
+  const msg = event.error ? String(event.error) : event.message
+  const stack = event.error && event.error.stack ? event.error.stack : ''
+  console.error('[Frontend Error]', msg, stack)
+  try {
+    if (window['go'] && window['go']['main'] && window['go']['main']['App'] && typeof window['go']['main']['App']['FrontendLog'] === 'function') {
+      window['go']['main']['App']['FrontendLog']('error', msg, stack)
+    }
+  } catch (e) {
+    // Backend not ready — ignore
+  }
 })
+
+window.addEventListener('unhandledrejection', (event) => {
+  const reason = event.reason
+  const msg = reason instanceof Error ? String(reason) : JSON.stringify(reason)
+  const stack = reason instanceof Error && reason.stack ? reason.stack : ''
+  console.error('[Unhandled Promise Rejection]', msg, stack)
+  try {
+    if (window['go'] && window['go']['main'] && window['go']['main']['App'] && typeof window['go']['main']['App']['FrontendLog'] === 'function') {
+      window['go']['main']['App']['FrontendLog']('error', 'Unhandled rejection: ' + msg, stack)
+    }
+  } catch (e) {
+    // Backend not ready — ignore
+  }
+})
+
+// Mount App with error fallback
+try {
+  new App({
+    target: document.getElementById('app')
+  })
+} catch (e) {
+  console.error('[Fatal] App mount failed:', e)
+  const el = document.getElementById('app')
+  if (el) {
+    el.innerHTML = '<div style="padding:24px;color:#ff6b6b;font-family:monospace;white-space:pre-wrap">'
+      + '<h2>Frontend Runtime Error</h2>'
+      + '<p>' + String(e) + '</p>'
+      + (e.stack ? '<pre>' + e.stack + '</pre>' : '')
+      + '</div>'
+  }
+}

frontend/src/wailsjs/go/main/App.js

@@ -243,5 +243,25 @@ export function CountActivityByNode(arg1) {
 }
 
 export function CreateEmptyFile(arg1, arg2) {
-    return window['go']['main']['App']['CreateEmptyFile'](arg1, arg2);
+    return window['go']['main']['App']['CreateEmptyFile'](arg1, arg2)
+}
+
+// ===== Logging & Diagnostics =====
+
+export function FrontendLog(level, message, stack) {
+    try {
+        if (window['go'] && window['go']['main'] && window['go']['main']['App']) {
+            return window['go']['main']['App']['FrontendLog'](level, message, stack || '')
+        }
+    } catch (e) {}
+    return Promise.resolve()
+}
+
+export function LogStartupStep(step, success, detail) {
+    try {
+        if (window['go'] && window['go']['main'] && window['go']['main']['App']) {
+            return window['go']['main']['App']['LogStartupStep'](step, success, detail || '')
+        }
+    } catch (e) {}
+    return Promise.resolve()
 }