Modernize launch screen, clean up dangling storyboard refs

[coffee-cup]

Replaces the 2016-era 375x667 launch image (letterboxed on modern devices) with a regenerated vector gradient using the app's own palette, and finishes the post-SwiftUI-migration project cleanup.

• New LaunchBackground.imageset: 1100x1500pt CoreGraphics-generated PDF, nauticalColour 0x4E50B8 (top) to risesetColour 0x46EAE7 (bottom) with plateaued ends — the full gradient is visible on every device from iPhone SE to iPad Pro 13" with no letterboxing (UILaunchScreen centers images at native size; UIImageRespectsSafeAreaInsets=false for edge-to-edge)
• Strip dangling Main/LaunchScreen storyboard references (empty PBXVariantGroups + Resources build files) left in the pbxproj after the storyboards were deleted in Modernize app: SwiftUI lifecycle, Swift 6, iOS 18 #5
• README: requirements line updated to Xcode 26+ / iOS 18+
• AGENTS.md: Swift 6 moved to done; new gotcha — SplashBoard caches launch screens per bundle ID across reinstalls (reboot the sim), and simctl launch --wait-for-debugger holds the launch screen for screenshots

Verified on the iPhone 17 Pro sim: settled launch screen is full-bleed gradient, normal launch hands off seamlessly to the timeline.