fix: address user feedback — AI loan categorization, invoice UX, email extension, supplier module

- #43: Improve AI categorization to use account 2350 for loan repayments
  instead of incorrectly suggesting 2440 (supplier payables). Add explicit
  prompt guidance distinguishing loans from supplier debts.
- #45: Change unclear invoice unit "mån" to "månad"
- #46: Enable email extension in extensions.config.json so it appears in
  the marketplace and can be activated by users
- #47: Change "Makulera" to "Ta bort utkast" for draft invoices — reserve
  "Makulera" terminology for proforma invoices only
- #48: Show field-level validation errors when supplier creation fails
  instead of generic "Validation failed" message
- #49: Temporarily hide Leverantörer and Leverantörsfakturor from sidebar
  pending module rework

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

lib/extensions/_generated/sector-definitions.ts

@@ -57,5 +57,20 @@ export const EXTENSION_DEFINITIONS: Record<string, ExtensionDefinition[]> = {
                 "event": "open-ai-chat"
           }
     },
+    {
+          "slug": "email",
+          "name": "E-post (Resend)",
+          "sector": "general",
+          "category": "operations",
+          "icon": "Mail",
+          "dataPattern": "core",
+          "description": "Skicka fakturor och påminnelser via e-post",
+          "longDescription": "Aktiverar e-postfunktioner: skicka fakturor till kunder, automatiska betalningspåminnelser (15/30/45 dagar), och e-postmeddelanden. Kräver ett Resend-konto med verifierad domän.",
+          "readsCoreTables": [
+                "invoices",
+                "customers",
+                "company_settings"
+          ]
+    },
   ],
 }