fix(player): make native Android video opt-in again — it shipped as audio with no picture

DR-161 flipped experimentalNativeVideo on by default so picture-in-picture could
shrink a real video surface. On a device that shipped sound with a blank screen.

The decode path was never at fault. Logcat shows ExoPlayer running and feeding a
live SurfaceView with an active BufferQueue. The compositing was: the SurfaceView
sits behind the WebView, and the step that clears the opaque layers above it
never took effect — `WebView transparent = false` is logged, `= true` never
appears. The video was rendering correctly the whole time, behind an opaque page.

This is precisely the defect the flag existed to contain;
VideoPlayer.scrubRegression.test.ts had already recorded that "the native
SurfaceView has never been visible through the webview". Enabling it by default
shipped a verified decode path on top of an unverified display path.

Reverting costs nothing that matters: PiP does not depend on it — DR-160 drives
PiP from the WebView <video> — and working video outranks PiP showing a native
surface. The flag stays in Settings, now described as incomplete rather than as a
performance win, so anyone helping test it still can.

Fixing the compositing is the prerequisite for trying this default again (DR-172).

src/routes/settings/+page.svelte

@@ -744,10 +744,10 @@
                 </h3>
                 <p class="text-sm text-gray-400 mt-1">
                   Decode video with the device's hardware decoder instead of the
-                  built-in web player. Better performance and battery life, and
-                  required for picture-in-picture to show the video rather than
-                  the app. Still less tested — turn this off if video fails to
-                  appear or seeking misbehaves.
+                  built-in web player. Better performance and battery life in
+                  principle, but incomplete: on some devices the picture does not
+                  appear at all and only the sound plays. Leave this off unless
+                  you are helping test it.
                 </p>
               </div>
               <button