Default a fresh install to 3:00 + 12s

The reserve was already three minutes; only the delay moves, 15s to 12s,
which is the control both of the other presets use.

It is a default, not a migration: store.num only falls back when nothing
is saved, so anyone already carrying a bg.delay keeps it. Checked both
ways — an empty profile comes up with 12s and the 3m/12s preset already
lit, a profile holding 15000 comes up unchanged with 3m/15s lit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

public_html/index.html

@@ -556,7 +556,7 @@
 
   var cfg = {
     base:   store.num("bg.base", 180000),      // reserve per point
-    delay:  store.num("bg.delay", 15000),
+    delay:  store.num("bg.delay", 12000),
     points: store.num("bg.points", 1),         // match length, odd, 1..29
     theme:  store.get("bg.theme", "sage"),
     sound:  store.get("bg.sound", "1") === "1"