Prevent splitter panels from collapsing to zero

Both splitters now have setChildrenCollapsible(False) so neither side
can be dragged to zero width. Right panel gets minWidth 400 to stay
usable when the source panel is expanded.

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

ui/main_window.py

@@ -1008,6 +1008,7 @@ class SequenceLinkerUI(QWidget):
         self.content_splitter.addWidget(file_list_panel)
         self.content_splitter.addWidget(self.preview_tabs)
         self.content_splitter.setHandleWidth(5)
+        self.content_splitter.setChildrenCollapsible(False)
         self.content_splitter.setSizes([350, 450])
 
         # Export buttons layout
@@ -1045,10 +1046,12 @@ class SequenceLinkerUI(QWidget):
         right_layout.addWidget(self.export_options_group)
 
         # === MAIN SPLITTER: Source Panel | Main Content ===
+        right_panel.setMinimumWidth(400)
         self.main_splitter = QSplitter(Qt.Orientation.Horizontal)
         self.main_splitter.addWidget(self.source_panel)
         self.main_splitter.addWidget(right_panel)
         self.main_splitter.setHandleWidth(5)
+        self.main_splitter.setChildrenCollapsible(False)
         self.main_splitter.setSizes([250, 750])
         self.main_splitter.setStretchFactor(0, 0)  # Source panel doesn't stretch
         self.main_splitter.setStretchFactor(1, 1)  # Main content stretches