Put the settings out of reach once a match is under way

Every control in the settings sheet calls resetClocks(), so opening it
mid-match could throw away two live clocks. The gear now only appears on an
untouched match: before the first tap, or after a two-tap reset.

"Untouched" is derived rather than tracked — phase, turn, moves, score and
both reserves matching matchTime() — so it cannot fall out of sync the way a
stored flag could. The phase alone would be wrong: a score change runs
newGame(), which returns to IDLE waiting for the next game's first tap while
the match is still very much on. The reserve check also catches a clock
corrected through the score sheet.

The button uses visibility rather than [hidden], so its slot stays put and
the rest of the bar doesn't jump the instant the clock starts; verified
pixel-for-pixel that reset, play/pause, score and sound do not move.
visibility:hidden also drops it from the tab order and the accessibility
tree. The rule is enforced in the click handler as well, because a
programmatic click still reaches a visibility:hidden button — CSS is the
affordance, the guard is the rule.

The trade, deliberately: a mistake in the time or delay now costs a reset and
a restarted match. Correcting a clock mid-match is the score sheet's job.

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-v7";
+const CACHE = "bgclock-v8";
 const FILES = [
   "./",
   "./index.html",