fix: increase surface contrast — cards now clearly separate from background

Dark mode:
  Canvas: #1a1714 → #0e0d0b (deeper black, more separation)
  Card:   #26231f → #1e1b17 (warmer, 7% brighter than bg)

Light mode:
  Canvas: #F5EFE6 → #EDE6DA (cooler sand, slightly darker)
  Card:   #FFFCF8 → #FFFFFF (clean white, max contrast)

Both modes now have ~7% brightness gap + temperature contrast
(warm bg, cleaner card). Cards visually float without needing
borders.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

ios/Platform/Platform/Shared/Extensions/Color+Extensions.swift

@@ -4,8 +4,8 @@ extension Color {
     // MARK: - Canvas / Background (adaptive light/dark)
     static let canvas = Color(UIColor { traits in
         traits.userInterfaceStyle == .dark
-            ? UIColor(red: 0.10, green: 0.09, blue: 0.08, alpha: 1) // warm dark
-            : UIColor(red: 0.96, green: 0.94, blue: 0.90, alpha: 1) // #F5EFE6
+            ? UIColor(red: 0.055, green: 0.05, blue: 0.04, alpha: 1) // #0e0d0b — deep warm black
+            : UIColor(red: 0.93, green: 0.90, blue: 0.855, alpha: 1) // #EDE6DA — warm sand
     })
 
     // MARK: - Accent
@@ -19,8 +19,8 @@ extension Color {
     // MARK: - Surfaces (adaptive)
     static let surfaceCard = Color(UIColor { traits in
         traits.userInterfaceStyle == .dark
-            ? UIColor(red: 0.15, green: 0.14, blue: 0.13, alpha: 1) // warm dark card
-            : UIColor(red: 1.0, green: 0.988, blue: 0.973, alpha: 1) // warm white
+            ? UIColor(red: 0.12, green: 0.11, blue: 0.09, alpha: 1) // #1e1b17 — warm dark card
+            : UIColor(red: 1.0, green: 1.0, blue: 1.0, alpha: 1)   // #FFFFFF — clean white
     })
     static let surfaceSheet = Color(UIColor { traits in
         traits.userInterfaceStyle == .dark