feat: open-source under AGPL-3.0, redesign UI to grayscale palette, add uncategorize API, fix VAT account names

Add LICENSE (AGPL-3.0-or-later), CONTRIBUTING.md, SECURITY.md, DCO, and NOTICE files.
Rewrite README for open-source audience with self-hosting instructions.
Redesign color palette to grayscale chrome theme across all components.
Add transaction uncategorize API route with tests.
Fix VAT account name mismatches in migration 052.
Improve import page with SIE file support and loading skeleton.

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

components/chat/artifacts/ChatAgingBuckets.tsx

@@ -4,11 +4,11 @@ import type { AgingBucketsArtifact } from '@/types/chat'
 import { formatCurrency } from '@/lib/utils'
 
 const BUCKET_COLORS = [
-  'bg-green-500',
-  'bg-yellow-400',
-  'bg-orange-400',
-  'bg-red-400',
-  'bg-red-600',
+  'bg-success',
+  'bg-warning',
+  'bg-warning/70',
+  'bg-destructive/70',
+  'bg-destructive',
 ]
 
 interface ChatAgingBucketsProps {

components/chat/artifacts/ChatKpiCards.tsx

@@ -26,9 +26,9 @@ export function ChatKpiCards({ artifact }: ChatKpiCardsProps) {
                 <span
                   className={`flex items-center gap-0.5 text-xs ${
                     card.trend === 'up'
-                      ? 'text-green-600 dark:text-green-400'
+                      ? 'text-success'
                       : card.trend === 'down'
-                        ? 'text-red-600 dark:text-red-400'
+                        ? 'text-destructive'
                         : 'text-muted-foreground'
                   }`}
                 >