Refactor financial transaction handling: Consolidate Einnahmen and Ausgaben into Buchung model, update routes and UI components, and add new migration scripts for database schema changes.

prisma/migrations/20260324_add_invoice_buchung_link/migration.sql

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+-- Add buchungId field to invoices table
+ALTER TABLE `invoices` ADD COLUMN `buchungId` VARCHAR(191) NULL;
+
+-- Create unique index for the foreign key
+ALTER TABLE `invoices` ADD UNIQUE INDEX `invoices_buchungId_key`(`buchungId`);
+
+-- Add foreign key constraint
+ALTER TABLE `invoices` ADD CONSTRAINT `invoices_buchungId_fkey`
+  FOREIGN KEY (`buchungId`) REFERENCES `buchungen`(`id`) ON DELETE SET NULL ON UPDATE CASCADE;