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/import/BankFileUploadStep.tsx

@@ -166,7 +166,7 @@ export default function BankFileUploadStep({
               </div>
             ) : selectedFile && detectedFormat ? (
               <div className="space-y-4">
-                <CheckCircle className="mx-auto h-12 w-12 text-green-600" />
+                <CheckCircle className="mx-auto h-12 w-12 text-success" />
                 <div>
                   <p className="font-medium">{selectedFile.name}</p>
                   <p className="text-sm text-muted-foreground">