feat(sharing): implement unified sharing system for social media

Replace old ProgressCardGenerator with protocol-based sharing architecture
supporting trips, achievements, and stadium progress. Features 8 color
themes, Instagram Stories optimization (1080x1920), and reusable card
components with map snapshots.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

SportsTime/Export/Sharing/ShareService.swift

@@ -0,0 +1,121 @@
+//
+//  ShareService.swift
+//  SportsTime
+//
+//  Handles Instagram direct share and fallback to system share sheet.
+//
+
+import SwiftUI
+import UIKit
+
+@MainActor
+final class ShareService {
+
+    static let shared = ShareService()
+
+    private init() {}
+
+    // MARK: - Share to Instagram
+
+    func shareToInstagram(image: UIImage) -> Bool {
+        guard let imageData = image.pngData() else { return false }
+
+        // Check if Instagram is installed
+        guard let instagramURL = URL(string: "instagram-stories://share"),
+              UIApplication.shared.canOpenURL(instagramURL) else {
+            return false
+        }
+
+        // Set up pasteboard with image
+        let pasteboardItems: [String: Any] = [
+            "com.instagram.sharedSticker.backgroundImage": imageData
+        ]
+
+        UIPasteboard.general.setItems(
+            [pasteboardItems],
+            options: [.expirationDate: Date().addingTimeInterval(300)]
+        )
+
+        // Open Instagram Stories
+        let urlString = "instagram-stories://share?source_application=com.sportstime.app"
+        if let url = URL(string: urlString) {
+            UIApplication.shared.open(url)
+            return true
+        }
+
+        return false
+    }
+
+    // MARK: - Copy to Clipboard
+
+    func copyToClipboard(image: UIImage) {
+        UIPasteboard.general.image = image
+    }
+
+    // MARK: - System Share Sheet
+
+    func presentShareSheet(image: UIImage, from viewController: UIViewController) {
+        let activityVC = UIActivityViewController(
+            activityItems: [image],
+            applicationActivities: nil
+        )
+
+        // iPad support
+        if let popover = activityVC.popoverPresentationController {
+            popover.sourceView = viewController.view
+            popover.sourceRect = CGRect(
+                x: viewController.view.bounds.midX,
+                y: viewController.view.bounds.midY,
+                width: 0,
+                height: 0
+            )
+        }
+
+        viewController.present(activityVC, animated: true)
+    }
+}
+
+// MARK: - Theme Persistence
+
+enum ShareThemePreferences {
+    private static let tripKey = "shareTheme.trip"
+    private static let achievementKey = "shareTheme.achievement"
+    private static let progressKey = "shareTheme.progress"
+
+    static var tripTheme: ShareTheme {
+        get { ShareTheme.theme(byId: UserDefaults.standard.string(forKey: tripKey) ?? "dark") }
+        set { UserDefaults.standard.set(newValue.id, forKey: tripKey) }
+    }
+
+    static var achievementTheme: ShareTheme {
+        get { ShareTheme.theme(byId: UserDefaults.standard.string(forKey: achievementKey) ?? "dark") }
+        set { UserDefaults.standard.set(newValue.id, forKey: achievementKey) }
+    }
+
+    static var progressTheme: ShareTheme {
+        get { ShareTheme.theme(byId: UserDefaults.standard.string(forKey: progressKey) ?? "dark") }
+        set { UserDefaults.standard.set(newValue.id, forKey: progressKey) }
+    }
+
+    static func theme(for cardType: ShareCardType) -> ShareTheme {
+        switch cardType {
+        case .tripSummary:
+            return tripTheme
+        case .achievementSpotlight, .achievementCollection, .achievementMilestone, .achievementContext:
+            return achievementTheme
+        case .stadiumProgress:
+            return progressTheme
+        }
+    }
+
+    static func setTheme(_ theme: ShareTheme, for cardType: ShareCardType) {
+        switch cardType {
+        case .tripSummary:
+            tripTheme = theme
+        case .achievementSpotlight, .achievementCollection, .achievementMilestone, .achievementContext:
+            achievementTheme = theme
+        case .stadiumProgress:
+            progressTheme = theme
+        }
+    }
+}