fix(android): keep the app out from under the system bars

Reported from the first device run: the playback controls are off
screen. `targetSdk 35` is Android 15, which lays every app out
edge-to-edge and ignores the deprecated `statusBarColor` and
`navigationBarColor` the scaffold's theme still sets -- so a
`match_parent` WebView draws the page's bottom band, which on a phone
is the transport *and* the tab bar, underneath the gesture bar.

`applyWindowInsets()` pads the container by
`systemBars | displayCutout | ime` and returns the insets rather than
consuming them, so the WebView is laid out inside them. The keyboard is
in the mask because a search box the keyboard covers is the same bug
one surface over.

The window background goes black to match the app's own default ramp:
that padding is what shows through, and a band of the scaffold's
blue-grey above and below reads as the app failing to fill the screen.

No tier we have can see this class of fault -- a browser viewport has
no system bars, so `phone-shell.spec.ts` at 390x844 renders a shell
that fits at the moment the device is clipping it. Verified only as far
as the APK building; the insets need the next build on a phone.

build/android/app/src/main/res/values/colors.xml

@@ -2,7 +2,12 @@
 <resources>
     <color name="wails_blue">#3574D4</color>
     <color name="wails_blue_dark">#2C5FB8</color>
-    <color name="wails_background">#1B2636</color>
+    <!-- The window background, which is what the launch screen shows and
+         what the system-bar padding leaves visible. Black rather than the
+         scaffold's blue-grey because this app's own default ramp is
+         black: a band of #1B2636 above and below it reads as the app
+         failing to fill the screen. -->
+    <color name="wails_background">#000000</color>
     <color name="white">#FFFFFFFF</color>
     <color name="black">#FF000000</color>
 </resources>