Lower deployment target from iOS 26 to iOS 18, gate Foundation Models behind @available

Broadens installable audience to iOS 18+ while keeping AI insights available on iOS 26.
Foundation Models types and service wrapped in @available(iOS 26, *), InsightsViewModel
conditionally instantiates the service with fallback UI on older versions.

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

Shared/Models/AIInsight.swift

@@ -9,6 +9,7 @@ import Foundation
 import FoundationModels
 
 /// Represents a single AI-generated insight using @Generable for structured LLM output
+@available(iOS 26, *)
 @Generable
 struct AIInsight: Equatable {
     @Guide(description: "A brief, engaging title for the insight (3-6 words)")
@@ -31,6 +32,7 @@ struct AIInsight: Equatable {
 }
 
 /// Container for period-specific insights - the LLM generates this structure
+@available(iOS 26, *)
 @Generable
 struct AIInsightsResponse: Equatable {
     @Guide(description: "Array of exactly 5 diverse insights covering patterns, advice, and predictions")
@@ -39,6 +41,7 @@ struct AIInsightsResponse: Equatable {
 
 // MARK: - Conversion to App's Insight Model
 
+@available(iOS 26, *)
 extension AIInsight {
     /// Converts AI-generated insight to the app's existing Insight model
     func toInsight() -> Insight {
@@ -60,6 +63,7 @@ extension AIInsight {
     }
 }
 
+@available(iOS 26, *)
 extension AIInsightsResponse {
     /// Converts all AI insights to app's Insight models
     func toInsights() -> [Insight] {

Shared/Models/UserDefaultsStore.swift

@@ -127,7 +127,7 @@ enum DayViewStyle: Int, CaseIterable {
     case wave = 13        // Horizontal gradient river bands
     case pattern = 14     // Mood icons as repeating background pattern
     case leather = 15     // Skeuomorphic leather with stitching
-    case glass = 16       // iOS 26 liquid glass with variable blur
+    case glass = 16       // Liquid glass with variable blur
     case motion = 17      // Accelerometer-driven parallax effect
     case micro = 18       // Ultra compact single-line entries
     case orbit = 19       // Celestial circular orbital arrangement