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/schemas/index.ts

@@ -73,14 +73,20 @@ export const StopTimerSchema = z.object({
   projectId: z.string().uuid().optional(),
 });
 
+const WorkingDayEnum = z.enum(['MON', 'TUE', 'WED', 'THU', 'FRI', 'SAT', 'SUN']);
+
 export const CreateClientTargetSchema = z.object({
   clientId: z.string().uuid(),
-  weeklyHours: z.number().positive().max(168),
+  targetHours: z.number().positive().max(168),
+  periodType: z.enum(['weekly', 'monthly']),
+  workingDays: z.array(WorkingDayEnum).min(1, 'At least one working day is required'),
   startDate: z.string().regex(/^\d{4}-\d{2}-\d{2}$/, 'startDate must be a date in YYYY-MM-DD format'),
 });
 
 export const UpdateClientTargetSchema = z.object({
-  weeklyHours: z.number().positive().max(168).optional(),
+  targetHours: z.number().positive().max(168).optional(),
+  periodType: z.enum(['weekly', 'monthly']).optional(),
+  workingDays: z.array(WorkingDayEnum).min(1, 'At least one working day is required').optional(),
   startDate: z.string().regex(/^\d{4}-\d{2}-\d{2}$/, 'startDate must be a date in YYYY-MM-DD format').optional(),
 });