fix: auto-pan timeline to follow playback position when zoomed in

Revert span opacity back to 35 (was fine). The actual issue was the
play position line disappearing when scrolled out of the zoomed view.
Now set_play_position auto-pans the view window to keep the playback
marker visible with a 10% margin.

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

main.py

@@ -1814,6 +1814,15 @@ class TimelineWidget(QWidget):
         if self._locked and self._play_pos is not None and self._seek_timer.isActive():
             return
         self._play_pos = t
+        if t is not None and self._view_span > 0:
+            view_end = self._view_start + self._view_span
+            margin = self._view_span * 0.1
+            if t > view_end - margin:
+                self._view_start = t - self._view_span + margin
+                self._clamp_view()
+            elif t < self._view_start + margin:
+                self._view_start = t - margin
+                self._clamp_view()
         self.update()
 
     def set_crop_keyframes(self, kfs: list[tuple[float, float, str | None, bool, bool]]) -> None:
@@ -2017,7 +2026,7 @@ class TimelineWidget(QWidget):
                 mx1 = int(self._time_to_x(t))
                 mx2 = int(self._time_to_x(min(t + span, self._duration)))
                 if mx2 > mx1 and mx2 > 0 and mx1 < w:
-                    p.fillRect(mx1, rh, mx2 - mx1, th, QColor(200, 160, 60, 70))
+                    p.fillRect(mx1, rh, mx2 - mx1, th, QColor(200, 160, 60, 35))
 
             # ── export markers ────────────────────────────────────────────
             p.setFont(self._marker_font)