Fix icon toggles, add timing presets and a two-tap reset

The play/pause and sound icons never toggled: SVGElement doesn't inherit
from HTMLElement, so it has no .hidden property and `svg.hidden = true`
only set a JS expando. Both icons then rendered at once because
`.btn svg{display:block}` out-specifies the UA's `[hidden]{display:none}`.
Fixed on both sides — a showIcon() helper sets the attribute, and the CSS
now honours [hidden]. Starting the clock by tapping a panel didn't repaint
the button either, which the working toggle made visible.

Also:
- conventional gear for the settings icon, replacing the clock glyph
- opening settings pauses the clock, and leaves it paused on close
- reset needs two taps within 2s, with a ring that drains over the window
- 2m/12s and 3m/15s presets atop the settings sheet; the highlight is
  derived from cfg, so it clears and returns as the steppers move
- ask for persistent storage so settings aren't evicted, and fall back to
  the defaults rather than NaN if a stored value is ever corrupt

sw.js cache bumped to v2 so installed devices pick up the new index.html.

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

public_html/sw.js

@@ -1,5 +1,5 @@
 /* Cache-first: once installed the clock never touches the network again. */
-const CACHE = "bgclock-v1";
+const CACHE = "bgclock-v2";
 const FILES = [
   "./",
   "./index.html",