Sync visible GPU duration slider default to Fast mode's 15s

templates/app.html still hardcoded the visible slider's initial value
at 30, and mode_toggle.js only applied the mode's duration on a
mode-button click, not on page load, so the UI kept showing 30s after
8a3b1b0 dropped the actual default to 15s.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

static/mode_toggle.js

@@ -21,4 +21,5 @@ window.__setMode = function(m) {
     applyModeSliderDefault('custom-gpu-duration', MODE_GPU_DURATION, m);
     applyModeSliderDefault('custom-steps', MODE_STEPS, m);
 };
+window.__setMode(window.__selectedMode);
 }

templates/app.html

@@ -120,8 +120,8 @@
                 <div class="settings-group-body">
                     <div class="slider-row">
                         <label>GPU時間（秒）</label>
-                        <input type="range" id="custom-gpu-duration" min="10" max="120" step="5" value="30">
-                        <span class="slider-val" id="custom-gpu-duration-val">30</span>
+                        <input type="range" id="custom-gpu-duration" min="10" max="120" step="5" value="15">
+                        <span class="slider-val" id="custom-gpu-duration-val">15</span>
                     </div>
                     <div class="slider-row">
                         <label>シード</label>