fix(nav): restore reference theme toggle geometry and single binding

The top-bar theme switch was bound twice — core/theme.js attached a click
handler to #theme-toggle-desktop while runtime/main.js delegated click on
.toggle-theme, and both ship in main.min.js, so one press toggled twice and
the button looked dead.

Also realign the control with the reference:

- Drop the far modifier from fa-toggle-theme so the sun/moon glyphs resolve
  against the Solid face at the reference weight instead of Regular.
- Restore .toggle-radius .fa/.icon to the reference form (left/right 0 +
  auto margin only). The extra position/top/transform/size/flex block and
  the > i selector were overriding the shared abs-* centering rule.
- Replace the per-ID bindings with one delegated listener so the floating
  rail, which carries no id, keeps working.
- Remove dead theme.js queries for selectors no markup or stylesheet ships
  (#theme-toggle, #dropdown-theme-toggle, .theme-icon-moon/.theme-icon-sun,
  [data-action='toggle-theme'], .is-dark).

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

assets/css/core/buttons.css

@@ -274,21 +274,10 @@
 }
 /* legacy groups these with abs-* : absolute + vertical center; then pin left/right 0 */
 .toggle-radius .fa,
-.toggle-radius .icon,
-.toggle-radius > i {
-  position: absolute;
-  top: 50%;
+.toggle-radius .icon {
   right: 0 !important;
   left: 0 !important;
   margin: auto;
-  transform: translateY(-50%);
-  line-height: 1;
-  width: 1em;
-  height: 1em;
-  display: inline-flex;
-  align-items: center;
-  justify-content: center;
-  text-align: center;
 }
 
 /* solid color modifiers — set --this-bg; default --this-color white */