feat(player): make native Android video the default

The two defects that were holding the flip back are fixed and verified on a
device, which is the standard this default has been held to since DR-161 shipped
a verified sub-path over an unverified one:

  - returning from background audio restarts the renderer that is actually on
    screen, instead of only ever reloading the <video> element (DR-196)
  - the letterbox bars are painted, instead of retaining whatever was last in
    the framebuffer (DR-194)

Evidence: handoff to audio-only at 69:54 returning to video playing at 70:18,
and clean bars across playback, the control bar and a rotation round-trip.

An explicit stored choice still wins in both directions, so anyone who turned the
flag off keeps it off — hence the null check on the stored value rather than a
bare === "true", which would silently re-enable it for people who opted out.

The Settings copy no longer tells users to leave it off; it now describes the
toggle as the fallback to the built-in web player.

The flag keeps its "experimental" name because it remains a suppressor of Rust's
backend choice, never a promoter: turning it on cannot produce a native backend
where Rust says HTML5.

src/routes/settings/+page.svelte

@@ -746,9 +746,8 @@
                   Decode video with the device's hardware decoder instead of the
                   built-in web player, for better performance and battery life,
                   and so picture-in-picture shows the video rather than the app.
-                  The picture works, but background audio does not come back from
-                  the lockscreen on this path yet — leave it off unless you are
-                  helping test it.
+                  On by default. Turn it off to fall back to the built-in web
+                  player if a video misbehaves.
                 </p>
               </div>
               <button