Arrow-key skip preserves play/pause state instead of always pausing

_seek_relative() was unconditionally calling player.pause() before
seeking. mpv's seek command doesn't itself change pause state, so
simply not forcing it is enough: paused stays paused, playing keeps
playing through the skip. (Frame-step still always pauses, correctly
-- that's for precisely inspecting a single frame, a different
use case from a coarse 5s nudge.)

Verified by calling _seek_relative() directly rather than simulating
the QShortcut keypress: a debug script isolated that PlayerWidget.seek()
itself works fine while playing, and QTest.keyClick against this
shortcut turned out to be unreliable in this headless setup even with
Qt's active-window state forced -- consistent with the flakiness
already noted in smoke_test_arrow_seek.py. The real key binding itself
was already confirmed with live xdotool keypresses in an earlier
session; this only needed to test the new state-preservation logic.

main.py

@@ -303,10 +303,13 @@ class MainWindow(QMainWindow):
         self.player.mpv.mute = checked
 
     def _seek_relative(self, delta_seconds: float):
+        # Unlike frame-stepping (which always pauses -- you're precisely
+        # inspecting a frame), a 5s skip is a coarse nudge while scrubbing
+        # around; seeking doesn't change mpv's pause state on its own, so
+        # just not touching play/pause here is enough to preserve whichever
+        # state the user was already in.
         if self.input_path is None:
             return
-        self.player.pause()
-        self.play_button.setText("Play")
         duration = self.player.mpv.duration or 0.0
         target = min(max(self.player.get_position() + delta_seconds, 0.0), duration)
         self.player.seek(target)