style: define missing --bg-elev-3 / --accent-fg CSS variables

--bg-elev-3 was referenced (spinner ring, search-clear hover) but never
defined in either theme, so var(--bg-elev-3) with no fallback produced an
invalid declaration — that's why the busy spinner ring was invisible
until it was switched to --border. Define it as a real elevation step
above --bg-elev-2 in dark (#34343f) and light (#e3e3dd).

Also define --accent-fg (#fff) explicitly instead of relying on the
inline var(--accent-fg, #fff) fallback.

Audited used-vs-defined custom properties: the only remaining "undefined"
one is --strength, which is set at runtime by JS on the password-strength
bar (has a 0% fallback) — intentional, not a bug.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

css/style.css

@@ -35,6 +35,7 @@ kbd {
     --bg:           #15151c;
     --bg-elev:      #1d1d26;
     --bg-elev-2:    #292932;
+    --bg-elev-3:    #34343f;   /* one step above elev-2 — hover / raised bits */
     --bg-glass:     rgba(29,29,38,0.78);
     --border:       #353541;
     --border-soft:  #25252e;
@@ -44,6 +45,7 @@ kbd {
     --accent:       #06b6d4;
     --accent-hover: #22d3ee;
     --accent-soft:  rgba(6,182,212,0.12);
+    --accent-fg:    #ffffff;   /* text/icon colour on an accent background */
     --accent-2:     #a78bfa;
     --danger:       #f43f5e;
     --danger-soft:  rgba(244,63,94,0.12);
@@ -63,6 +65,7 @@ kbd {
     --bg:           #f7f7f5;
     --bg-elev:      #ffffff;
     --bg-elev-2:    #efefec;
+    --bg-elev-3:    #e3e3dd;   /* one step above elev-2 — hover / raised bits */
     --bg-glass:     rgba(255,255,255,0.92);
     --border:       #dcdcd7;
     --border-soft:  #ebebe6;