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/schema.prisma

@@ -99,13 +99,20 @@ model OngoingTimer {
   @@map("ongoing_timers")
 }
 
+enum PeriodType {
+  WEEKLY
+  MONTHLY
+}
+
 model ClientTarget {
-  id          String    @id @default(uuid())
-  weeklyHours Float     @map("weekly_hours")
-  startDate   DateTime  @map("start_date") @db.Date // Always a Monday
-  createdAt   DateTime  @default(now()) @map("created_at")
-  updatedAt   DateTime  @updatedAt @map("updated_at")
-  deletedAt   DateTime? @map("deleted_at")
+  id          String     @id @default(uuid())
+  targetHours Float      @map("target_hours")
+  periodType  PeriodType @default(WEEKLY) @map("period_type")
+  workingDays String[]   @map("working_days") // e.g. ["MON","WED","FRI"]
+  startDate   DateTime   @map("start_date") @db.Date
+  createdAt   DateTime   @default(now()) @map("created_at")
+  updatedAt   DateTime   @updatedAt @map("updated_at")
+  deletedAt   DateTime?  @map("deleted_at")
 
   userId   String @map("user_id") @db.VarChar(255)
   user     User   @relation(fields: [userId], references: [id], onDelete: Cascade)