fix: prevent accidental seek on tap in mobile progress bar

Touch scrubbing now requires ≥8px horizontal drag before committing.
A plain tap on the progress bar is ignored — no unintentional position jump.
Also removed redundant onclick="seekTo()" since _initBarScrub already
handles mouse seeking via mousedown/mousemove/mouseup.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

index.html

@@ -436,7 +436,7 @@
       </div>
       <div class="progress-bar-wrap">
         <span class="progress-time" id="time-cur">0:00</span>
-        <div class="progress-bar" id="progress-bar" onclick="seekTo(event)"><div class="progress-fill" id="progress-fill"></div></div>
+        <div class="progress-bar" id="progress-bar"><div class="progress-fill" id="progress-fill"></div></div>
         <span class="progress-time right" id="time-total">0:00</span>
       </div>
     </div>