feat: implement monthly targets and work-day-aware balance calculation

Add support for monthly and weekly targets with work-day selection:
- Users can now set targets as 'monthly' or 'weekly'
- Users select which days of the week they work
- Balance is calculated per working day, evenly distributed
- Target hours = (periodHours / workingDaysInPeriod)
- Corrections are applied at period level
- Daily balance tracking replaces weekly granularity

Database changes:
- Rename weekly_hours -> target_hours
- Add period_type (weekly|monthly, default=weekly)
- Add work_days (integer array of ISO weekdays, default=[1,2,3,4,5])
- Add constraints for valid period_type and non-empty work_days

Backend:
- Rewrite balance calculation in ClientTargetService
- Support monthly period enumeration
- Calculate per-day targets based on selected working days
- Update Zod schemas for new fields
- Update TypeScript types

Frontend:
- Add period type selector in target form (weekly/monthly)
- Add work days multi-checkbox selector
- Conditional start date input (week vs month)
- Update DashboardPage to show 'this period' instead of 'this week'
- Update ClientsPage to display working days and period type
- Support both weekly and monthly targets in UI

Migration:
- Add migration to extend client_targets table with new columns
- Backfill existing targets with default values (weekly, Mon-Fri)

backend/prisma/migrations/20260224120000_extend_targets_with_period_and_workdays/migration.sql

@@ -0,0 +1,17 @@
+-- Extend client_targets with period_type and work_days, rename weekly_hours to target_hours
+-- This migration adds support for monthly targets and work-day-aware balance calculation
+
+-- Rename weekly_hours to target_hours
+ALTER TABLE "client_targets" RENAME COLUMN "weekly_hours" TO "target_hours";
+
+-- Add period_type column with default 'weekly' for backwards compatibility
+ALTER TABLE "client_targets" ADD COLUMN "period_type" VARCHAR(10) NOT NULL DEFAULT 'weekly';
+
+-- Add work_days column with default Mon-Fri (1-5) for backwards compatibility
+ALTER TABLE "client_targets" ADD COLUMN "work_days" INTEGER[] NOT NULL DEFAULT '{1,2,3,4,5}';
+
+-- Add check constraint to ensure period_type is valid
+ALTER TABLE "client_targets" ADD CONSTRAINT "period_type_check" CHECK ("period_type" IN ('weekly', 'monthly'));
+
+-- Add check constraint to ensure at least one work day is selected
+ALTER TABLE "client_targets" ADD CONSTRAINT "work_days_not_empty_check" CHECK (array_length("work_days", 1) > 0);