Ensure full Spanish localization for all 1.2.0 strings

- OnboardingView: wrap page titles/descriptions with String(localized:)
  using symbolic keys so SwiftUI can look them up correctly
- IAPService.paywallBenefits: change from static let to static var,
  wrap all strings with String(localized:) using symbolic keys
- en.lproj: add symbolic keys for new onboarding pages and paywall benefits
- es-ES.lproj: add Spanish translations for all new keys (onboarding pages,
  paywall benefits, batch update, contribution fields) plus pre-existing
  gaps (Monthly Highlights, Best/Worst Performer, Best Contributor)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

PortfolioJournal/Views/Onboarding/OnboardingView.swift

@@ -15,26 +15,26 @@ struct OnboardingView: View {
     private let pages: [OnboardingPage] = [
         OnboardingPage(
             icon: "chart.line.uptrend.xyaxis",
-            title: "Know exactly where you stand",
-            description: "See your total wealth, real returns, and allocation — always up to date.",
+            title: String(localized: "onboarding_clarity_title"),
+            description: String(localized: "onboarding_clarity_desc"),
             color: .appPrimary
         ),
         OnboardingPage(
             icon: "checkmark.circle.fill",
-            title: "5 minutes a month is enough",
-            description: "Log your values once a month. Portfolio Journal handles the math and shows your progress.",
+            title: String(localized: "onboarding_habit_title"),
+            description: String(localized: "onboarding_habit_desc"),
             color: .positiveGreen
         ),
         OnboardingPage(
             icon: "leaf.fill",
-            title: "Ignore the noise. Track the trend.",
-            description: "Daily swings don't tell the real story. Your growth over months and years does.",
+            title: String(localized: "onboarding_calm_title"),
+            description: String(localized: "onboarding_calm_desc"),
             color: .appSecondary
         ),
         OnboardingPage(
             icon: "flag.checkered",
-            title: "Reach your financial goals",
-            description: "Set targets, track milestones, and see exactly how far you've come.",
+            title: String(localized: "onboarding_goals_title"),
+            description: String(localized: "onboarding_goals_desc"),
             color: .appWarning
         )
     ]