fix: downgrade missing extension env vars to warning, add error boundaries and cron auth fixes

- Change missing extension env vars from throw to log.warn (graceful degradation)
- Move initialized flag after all init steps complete
- Add error.tsx and loading.tsx for dashboard error boundaries
- Fix cron route auth header checks
- Add ensureInitialized() to journal entry reverse and invoice mark-paid/sent routes

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

lib/init.ts

@@ -46,7 +46,7 @@ function validateEnvironment(): void {
   }
 
   if (missingExt.length > 0) {
-    throw new Error(`Missing required extension environment variables: ${missingExt.join(', ')}`)
+    log.warn(`Missing extension environment variables (extensions needing them may not work): ${missingExt.join(', ')}`)
   }
 
   for (const v of OPTIONAL_VARS) {
@@ -64,10 +64,11 @@ function validateEnvironment(): void {
  */
 export function ensureInitialized(): void {
   if (initialized) return
-  initialized = true
 
   validateEnvironment()
   setContextFactory(createExtensionContext)
   registerSupplierInvoiceHandler()
   loadExtensions()
+
+  initialized = true
 }