fix(v0.5.2-prep): replace retired BAGeL Radio with PopTron in QUICK_IDS

BAGeL Radio has been retired from SomaFM (no longer in their public
channels.json). The render loop drops missing slugs silently, so the
chip just disappeared — David flagged 'stations look a bit off' and
the screenshot showed 13 chips instead of 14, wrapping awkwardly as
6+6+1 with ThistleRadio alone.

PopTron (alternative / alt-rock) fills the same indie/alt-rock genre
slot that BAGeL covered and is currently a live SomaFM channel.

One-line edit.

src/newtab/newtab.js

@@ -75,7 +75,7 @@ const QUICK_IDS = [
   'beatblender',     // electronic / breakbeat
   'bootliquor',      // americana / outlaw country
   'u80s',            // 80s indie / synthwave (Underground 80s)
-  'bagel',           // indie / alt-rock (BAGeL Radio)
+  'poptron',         // alternative / alt-rock (PopTron) — replaces BAGeL Radio (retired by SomaFM)
   'thistle',         // celtic / folk (ThistleRadio)
 ];