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>

supabase/migrations/20240101000052_fix_vat_account_names.sql

@@ -0,0 +1,15 @@
+-- Migration 52: Fix incorrect VAT account names for existing users
+--
+-- Migration 009 seeded accounts 2610/2611/2612 with wrong names:
+--   2610 → "Utgaende moms 25%" (should not exist; 2611 is the correct account)
+--   2611 → "Utgaende moms 12%" (WRONG — 2611 is 25%, 2621 is 12%)
+--   2612 → "Utgaende moms 6%"  (WRONG — 2612 doesn't exist in BAS, 2631 is 6%)
+--
+-- Migration 021 fixed the seed function for new users but didn't fix existing data.
+-- This migration corrects the account_name for any existing rows.
+
+-- Fix 2611: was incorrectly labeled as 12%, should be 25%
+UPDATE public.chart_of_accounts
+SET account_name = 'Utgaende moms forsaljning 25%', updated_at = now()
+WHERE account_number = '2611'
+  AND account_name IN ('Utgaende moms 12%', 'Utgående moms 12%');