feat: implement client targets v2 (weekly/monthly periods, working days, pro-ration)

- Add PeriodType enum and working_days column to ClientTarget schema
- Rename weekly_hours -> target_hours; remove Monday-only constraint
- Add migration 20260224000000_client_targets_v2
- Rewrite computeBalance() to support weekly/monthly periods, per-spec
  pro-ration for first period, ongoing vs completed period logic, and
  elapsed working-day counting (§4–§6 of requirements doc)
- Update Zod schemas and TypeScript input types for new fields
- Frontend: replace WeekBalance with PeriodBalance; update
  ClientTargetWithBalance to currentPeriod* fields
- ClientTargetPanel: period type radio, working-day toggles, free date
  picker, dynamic hours label
- DashboardPage: rename widget to Targets, dynamic This week/This month
  label

backend/prisma/migrations/20260224000000_client_targets_v2/migration.sql

@@ -0,0 +1,10 @@
+-- CreateEnum
+CREATE TYPE "PeriodType" AS ENUM ('WEEKLY', 'MONTHLY');
+
+-- AlterTable: rename weekly_hours -> target_hours, add period_type, add working_days
+ALTER TABLE "client_targets"
+  RENAME COLUMN "weekly_hours" TO "target_hours";
+
+ALTER TABLE "client_targets"
+  ADD COLUMN "period_type" "PeriodType" NOT NULL DEFAULT 'WEEKLY',
+  ADD COLUMN "working_days" TEXT[] NOT NULL DEFAULT ARRAY['MON','TUE','WED','THU','FRI']::TEXT[];