fix(ereader): restore cover position when navigating back to the cover

previous_page() set the on-cover flag but left current_position at the
first content block, so "showing the cover" had two different internal
representations depending on how you got there: block 0 on a fresh load,
block 1 after going forward and back.

current_position is what gets persisted, so closing the book while on the
cover reopened it past the cover, silently losing it.

Reset current_position to block 0 when returning to the cover, matching
where a fresh load sits.

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

pyWebLayout/layout/ereader_manager.py

@@ -453,6 +453,12 @@ class EreaderLayoutManager:
         # Special case: if at the beginning of content and there's a cover, go back to it
         if self._has_cover and self._is_at_beginning() and not self._on_cover_page:
             self._on_cover_page = True
+            # Restore the canonical cover position. Being on the cover must have a
+            # single representation: a fresh load sits at block 0 with the cover
+            # showing, so returning to the cover has to land there too. Leaving the
+            # position at the first content block saves a position that reopens past
+            # the cover, silently losing it.
+            self.current_position = RenderingPosition()
             self._notify_position_changed()
             return self.get_current_page()