Add position-based colorful Home shortcut palettes

Colorful shortcut cards now color by grid position instead of shortcut
identity, so colors stay in the same slot when shortcuts are reordered.

Adds a Palette option (Classic default, plus Sunset, Meadow, Berry,
Grape) and a Custom palette with a swatch-list editor (ColorPicker + hex
fields) and a switchable comma-separated text field. Color is resolved
per position and wraps by palette length, wired via a new
homeShortcutColorfulColor environment value and a Color(hex:) extension.

Yattee/Extensions/Color+Hex.swift

@@ -0,0 +1,71 @@
+//
+//  Color+Hex.swift
+//  Yattee
+//
+//  Hex-string parsing/formatting for SwiftUI Color, plus the environment value
+//  used to drive position-based colorful shortcut cards.
+//
+
+import SwiftUI
+
+extension Color {
+    /// Creates a color from a `#RRGGBB` / `RRGGBB` hex string (also accepts
+    /// `#RGB` shorthand and an optional `AA`/`AARRGGBB`/`RRGGBBAA` alpha).
+    /// Returns `nil` for anything it can't parse so bad input is simply skipped.
+    init?(hex: String) {
+        var cleaned = hex.trimmingCharacters(in: .whitespacesAndNewlines)
+        if cleaned.hasPrefix("#") {
+            cleaned.removeFirst()
+        }
+        guard !cleaned.isEmpty, cleaned.allSatisfy({ $0.isHexDigit }) else { return nil }
+
+        // Expand #RGB shorthand to #RRGGBB.
+        if cleaned.count == 3 {
+            cleaned = cleaned.map { "\($0)\($0)" }.joined()
+        }
+
+        guard let value = UInt64(cleaned, radix: 16) else { return nil }
+
+        let r, g, b, a: Double
+        switch cleaned.count {
+        case 6: // RRGGBB
+            r = Double((value & 0xFF0000) >> 16) / 255
+            g = Double((value & 0x00FF00) >> 8) / 255
+            b = Double(value & 0x0000FF) / 255
+            a = 1
+        case 8: // RRGGBBAA
+            r = Double((value & 0xFF00_0000) >> 24) / 255
+            g = Double((value & 0x00FF_0000) >> 16) / 255
+            b = Double((value & 0x0000_FF00) >> 8) / 255
+            a = Double(value & 0x0000_00FF) / 255
+        default:
+            return nil
+        }
+
+        self.init(red: r, green: g, blue: b, opacity: a)
+    }
+
+    /// Formats the resolved color as an uppercase `#RRGGBB` string.
+    func toHexString() -> String {
+        let resolved = resolve(in: EnvironmentValues())
+        let r = Int((max(0, min(1, resolved.red)) * 255).rounded())
+        let g = Int((max(0, min(1, resolved.green)) * 255).rounded())
+        let b = Int((max(0, min(1, resolved.blue)) * 255).rounded())
+        return String(format: "#%02X%02X%02X", r, g, b)
+    }
+}
+
+// MARK: - Environment: position-based colorful color
+
+private struct HomeShortcutColorfulColorKey: EnvironmentKey {
+    static let defaultValue: Color? = nil
+}
+
+extension EnvironmentValues {
+    /// The colorful-style fill color resolved from the shortcut's grid position.
+    /// When set, it overrides a card's fixed `colorfulColor`.
+    var homeShortcutColorfulColor: Color? {
+        get { self[HomeShortcutColorfulColorKey.self] }
+        set { self[HomeShortcutColorfulColorKey.self] = newValue }
+    }
+}