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/src/types/index.ts

@@ -82,13 +82,17 @@ export interface StopTimerInput {
 
 export interface CreateClientTargetInput {
   clientId: string;
-  weeklyHours: number;
-  startDate: string; // YYYY-MM-DD, always a Monday
+  targetHours: number;
+  periodType: 'weekly' | 'monthly';
+  workingDays: string[]; // e.g. ["MON","WED","FRI"]
+  startDate: string; // YYYY-MM-DD
 }
 
 export interface UpdateClientTargetInput {
-  weeklyHours?: number;
-  startDate?: string; // YYYY-MM-DD, always a Monday
+  targetHours?: number;
+  periodType?: 'weekly' | 'monthly';
+  workingDays?: string[];
+  startDate?: string; // YYYY-MM-DD
 }
 
 export interface CreateCorrectionInput {