Root cause: server pause/resume commands send None for position and
tracks, which the renderer state blindly overwrote. Then renderer
reports sent null position to the server, making controllers show the
track as finished.
Three-part fix:
1. renderer.rs: only overwrite position/tracks when server provides
a non-None value (retain last known values through pause/resume)
2. app.rs + qconnect_service.rs: update renderer state position from
frontend periodic reports so the internal state always has the
real playback position
3. +page.svelte: increase position report frequency from 5s to 2s
for smoother seekbar updates on controllers
vicrodh