ui: default Background Particles OFF (#935)

the full-page particle canvas runs a continuous requestAnimationFrame loop behind every
page — real GPU cost, and multiple users hit GPU strain until they found the toggle. flip
the default to off; the eye candy is opt-in now.

- init.js: runtime flag defaults false unless localStorage is explicitly 'true'
- settings.js: config read is now '=== true' (default off) instead of '!== false'
- index.html: checkbox no longer 'checked' by default; hint reworded

existing users who explicitly enabled it (localStorage/config 'true') keep it on; the
existing '!== false' runtime guards still work since the flag is now always set explicitly.

webui/index.html

@@ -6263,10 +6263,10 @@
                                 </div>
                                 <div class="form-group">
                                     <label class="checkbox-label">
-                                        <input type="checkbox" id="particles-enabled" checked>
+                                        <input type="checkbox" id="particles-enabled">
                                         Background Particles
                                     </label>
-                                    <small class="settings-hint">Animated particle effects behind each page. Disable to reduce GPU usage.</small>
+                                    <small class="settings-hint">Animated particle effects behind each page. Off by default — enable for the eye candy (uses GPU).</small>
                                 </div>
                                 <div class="form-group">
                                     <label class="checkbox-label">

webui/static/init.js

@@ -260,10 +260,11 @@ function applyReduceEffects(enabled) {
     }
     const saved = localStorage.getItem('soulsync-accent');
     if (saved) applyAccentColor(saved);
-    // Bootstrap particles setting from localStorage
+    // Bootstrap particles setting from localStorage — OFF by default (continuous
+    // full-page canvas = real GPU cost); only on when the user explicitly enabled it.
     const particlesSaved = localStorage.getItem('soulsync-particles');
-    if (particlesSaved === 'false') {
-        window._particlesEnabled = false;
+    window._particlesEnabled = (particlesSaved === 'true');
+    if (!window._particlesEnabled) {
         const canvas = document.getElementById('page-particles-canvas');
         if (canvas) canvas.style.display = 'none';
     }

webui/static/settings.js

@@ -1387,7 +1387,7 @@ async function loadSettingsData() {
         sidebarVisualizerType = vizType;
 
         // Background particles toggle
-        const particlesEnabled = settings.ui_appearance?.particles_enabled !== false; // default true
+        const particlesEnabled = settings.ui_appearance?.particles_enabled === true; // default OFF (GPU cost)
         const particlesCheckbox = document.getElementById('particles-enabled');
         if (particlesCheckbox) particlesCheckbox.checked = particlesEnabled;
         applyParticlesSetting(particlesEnabled);