Add break time feature to time entries

- Add breakMinutes field to TimeEntry model and database migration
- Users can now add break duration (minutes) to time entries
- Break time is subtracted from total tracked duration
- Validation ensures break time cannot exceed total entry duration
- Statistics and client target balance calculations account for breaks
- Frontend UI includes break time input in TimeEntryFormModal
- Duration displays show break time deduction (e.g., '7h (−1h break)')
- Both project/client statistics and weekly balance calculations updated

backend/prisma/migrations/20260223123527_add_break_minutes/migration.sql

@@ -0,0 +1,2 @@
+-- AlterTable
+ALTER TABLE "time_entries" ADD COLUMN "break_minutes" INTEGER NOT NULL DEFAULT 0;