fix(highway): apply 3D fret-spacing live instead of reloading (#561) (#562)

window.h3dSetFretSpacing was the only 3D-highway setting that applied via
location.reload(). The SPA boots with #home as the active screen and has
no restore-last-screen mechanism, so the reload ejected the user from
Settings onto the home screen.

Apply it live like every other 3D-highway setting: rebind the module-scope
_h3dFretUniform flag (so panels mounted later this session pick up the new
mode), recompute the two fretX-derived scalars baked at init
(_fretLabelScaleRefW, FRET_WIDTH_MID), and broadcast a 'fretSpacing' change
over the existing _bgEmitChange pub-sub so every mounted panel rebuilds its
board via buildBoard(). Per-frame note geometry already reads fretX live.

Settings copy updated (no longer reloads) and tests/js pin the no-reload /
live-rebuild behavior.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/highway_3d/settings.html

@@ -52,7 +52,7 @@
             <option value="uniform">Uniform (equal width per fret)</option>
             <option value="logarithmic">Instrument (logarithmic — decreasing toward bridge)</option>
         </select>
-        <p class="text-xs text-gray-500 mt-1">The page will reload when changed.</p>
+        <p class="text-xs text-gray-500 mt-1">Applies to the 3D highway immediately.</p>
     </div>
 
     <script>