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/ShareableContent.swift

@@ -0,0 +1,157 @@
+//
+//  ShareableContent.swift
+//  SportsTime
+//
+//  Protocol for shareable content and theme definitions.
+//
+
+import SwiftUI
+import UIKit
+
+// MARK: - Shareable Content Protocol
+
+protocol ShareableContent {
+    var cardType: ShareCardType { get }
+    func render(theme: ShareTheme) async throws -> UIImage
+}
+
+// MARK: - Card Types
+
+enum ShareCardType: String, CaseIterable {
+    case tripSummary
+    case achievementSpotlight
+    case achievementCollection
+    case achievementMilestone
+    case achievementContext
+    case stadiumProgress
+}
+
+// MARK: - Share Theme
+
+struct ShareTheme: Identifiable, Hashable {
+    let id: String
+    let name: String
+    let gradientColors: [Color]
+    let accentColor: Color
+    let textColor: Color
+    let secondaryTextColor: Color
+    let useDarkMap: Bool
+
+    // MARK: - Preset Themes
+
+    static let dark = ShareTheme(
+        id: "dark",
+        name: "Dark",
+        gradientColors: [Color(hex: "1A1A2E"), Color(hex: "16213E")],
+        accentColor: Color(hex: "FF6B35"),
+        textColor: .white,
+        secondaryTextColor: Color(hex: "B8B8D1"),
+        useDarkMap: true
+    )
+
+    static let light = ShareTheme(
+        id: "light",
+        name: "Light",
+        gradientColors: [.white, Color(hex: "F5F5F5")],
+        accentColor: Color(hex: "FF6B35"),
+        textColor: Color(hex: "1A1A2E"),
+        secondaryTextColor: Color(hex: "666666"),
+        useDarkMap: false
+    )
+
+    static let midnight = ShareTheme(
+        id: "midnight",
+        name: "Midnight",
+        gradientColors: [Color(hex: "0D1B2A"), Color(hex: "1B263B")],
+        accentColor: Color(hex: "00D4FF"),
+        textColor: .white,
+        secondaryTextColor: Color(hex: "A0AEC0"),
+        useDarkMap: true
+    )
+
+    static let forest = ShareTheme(
+        id: "forest",
+        name: "Forest",
+        gradientColors: [Color(hex: "1B4332"), Color(hex: "2D6A4F")],
+        accentColor: Color(hex: "95D5B2"),
+        textColor: .white,
+        secondaryTextColor: Color(hex: "B7E4C7"),
+        useDarkMap: false
+    )
+
+    static let sunset = ShareTheme(
+        id: "sunset",
+        name: "Sunset",
+        gradientColors: [Color(hex: "FF6B35"), Color(hex: "F7931E")],
+        accentColor: .white,
+        textColor: .white,
+        secondaryTextColor: Color(hex: "FFE5D9"),
+        useDarkMap: false
+    )
+
+    static let berry = ShareTheme(
+        id: "berry",
+        name: "Berry",
+        gradientColors: [Color(hex: "4A0E4E"), Color(hex: "81267E")],
+        accentColor: Color(hex: "FF85A1"),
+        textColor: .white,
+        secondaryTextColor: Color(hex: "E0B0FF"),
+        useDarkMap: true
+    )
+
+    static let ocean = ShareTheme(
+        id: "ocean",
+        name: "Ocean",
+        gradientColors: [Color(hex: "023E8A"), Color(hex: "0077B6")],
+        accentColor: Color(hex: "90E0EF"),
+        textColor: .white,
+        secondaryTextColor: Color(hex: "CAF0F8"),
+        useDarkMap: true
+    )
+
+    static let slate = ShareTheme(
+        id: "slate",
+        name: "Slate",
+        gradientColors: [Color(hex: "2B2D42"), Color(hex: "3D405B")],
+        accentColor: Color(hex: "F4A261"),
+        textColor: Color(hex: "EDF2F4"),
+        secondaryTextColor: Color(hex: "8D99AE"),
+        useDarkMap: true
+    )
+
+    static let all: [ShareTheme] = [.dark, .light, .midnight, .forest, .sunset, .berry, .ocean, .slate]
+
+    static func theme(byId id: String) -> ShareTheme {
+        all.first { $0.id == id } ?? .dark
+    }
+}
+
+// MARK: - Share Errors
+
+enum ShareError: Error, LocalizedError {
+    case renderingFailed
+    case mapSnapshotFailed
+    case instagramNotInstalled
+
+    var errorDescription: String? {
+        switch self {
+        case .renderingFailed:
+            return "Failed to render share card"
+        case .mapSnapshotFailed:
+            return "Failed to generate map snapshot"
+        case .instagramNotInstalled:
+            return "Instagram is not installed"
+        }
+    }
+}
+
+// MARK: - Card Dimensions
+
+enum ShareCardDimensions {
+    static let cardSize = CGSize(width: 1080, height: 1920)
+    static let mapSnapshotSize = CGSize(width: 1000, height: 500)
+    static let routeMapSize = CGSize(width: 1000, height: 600)
+    static let padding: CGFloat = 60
+    static let headerHeight: CGFloat = 120
+    static let footerHeight: CGFloat = 100
+}