Better splash screens: iOS SVG + continuous native Android splash

[unlocdavid]

Full disclosure: This PR is made with Claude Opus

I had some issues with splash screens. On Android the splash was fuzzy in some cases, maybe because of my UI scaling. And the native Android splash and NativePHP splash generated visual noise on boot.

I investigated if SVG would be an option, but it seems to only be reliable for iOS. So this PR adds support for splash.svg and splash-dark.svg only on iOS. If an SVG is found, it takes precedence over PNGs.

On Android 12+ there is a native splash screen of the app icon in a circle on a colored background. After the first loading of the app it jumped to the NativePHP configured PNG splash which waits for PHP etc. boots. This PR adds the option to hold the native splash and don't show the PNG. This solves two problems:

• No fuzzy splash screens.
• No jarring jump between splash screens. Especially in a release build, non-first boots can be very fast (yay!) but jumping between the two screens generates a lot of visual noise for the user. Using native in the config eliminates the jump.

Below is the description generated by Claude:

---

Summary

Two splash-screen improvements:

• iOS accepts a native vector splash.svg (no rasterizer).
• Android holds the system splash until the WebView is ready, eliminating the
  unbranded-system-splash → custom-overlay jump, with a new config to choose the style.

Android splash input remains PNG-only (an SVG→VectorDrawable path was explored and dropped as too unreliable).

iOS

• public/splash.svg (+ optional splash-dark.svg) is written into LaunchImage.imageset
  as a vector asset (preserves-vector-representation). Invalid/missing SVG falls back to
  the existing PNG pipeline.

Android

• Add androidx.core:core-splashscreen; installSplashScreen() +
  setKeepOnScreenCondition hold the system splash through boot in native style.
• Generate Theme.AndroidPHP.Splash and point the app theme at it.
• New android.splash config: style (image default | native), background,
  background_night (env: NATIVEPHP_ANDROID_SPLASH_*).
• The system splash shows the app launcher icon in both styles. image keeps the
  full-bleed Compose overlay; native relies on the held system splash on API 31+, with
  the overlay as the fallback below 31.
• Splash style/icon/background are applied at build time, so config changes take
  effect on rebuild (not just install).
• Post-splash windowBackground tracks the splash color to avoid a white flash on handoff.
• A @drawable/splash drawable is always guaranteed (shipped placeholder + app-icon/PNG
  fallback) so resource linking never fails.

Config

'splash' => [
    'style' => env('NATIVEPHP_ANDROID_SPLASH_STYLE', 'image'),
    'background' => env('NATIVEPHP_ANDROID_SPLASH_BACKGROUND', '#FFFFFF'),
    'background_night' => env('NATIVEPHP_ANDROID_SPLASH_BACKGROUND_NIGHT', '#000000'),
],

Old configs without this block fall back to image mode with white/black defaults.