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>

app/api/bookkeeping/journal-entries/[id]/reverse/__tests__/route.test.ts

@@ -12,6 +12,10 @@ vi.mock('@/lib/supabase/server', () => ({
   createClient: () => mockCreateClient(),
 }))
 
+vi.mock('@/lib/init', () => ({
+  ensureInitialized: vi.fn(),
+}))
+
 const mockReverseEntry = vi.fn()
 vi.mock('@/lib/bookkeeping/engine', () => ({
   reverseEntry: (...args: unknown[]) => mockReverseEntry(...args),

app/api/bookkeeping/journal-entries/[id]/reverse/route.ts

@@ -1,6 +1,9 @@
 import { createClient } from '@/lib/supabase/server'
 import { NextResponse } from 'next/server'
 import { reverseEntry } from '@/lib/bookkeeping/engine'
+import { ensureInitialized } from '@/lib/init'
+
+ensureInitialized()
 
 export async function POST(
   request: Request,