Fix source panel splitter drag: remove maxWidth clamp, widen handles

The source panel had maxWidth(400) which barely let the splitter move
beyond its starting position. Removed the cap, lowered minWidth from
250 to 150, and widened both splitter handles to 5px for easier grab.

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

ui/main_window.py

@@ -451,8 +451,7 @@ class SequenceLinkerUI(QWidget):
         """Create all UI widgets."""
         # Source folders panel - side panel with single unified list
         self.source_panel = QWidget()
-        self.source_panel.setMinimumWidth(250)
-        self.source_panel.setMaximumWidth(400)
+        self.source_panel.setMinimumWidth(150)
 
         # Single unified source list (odd=Main, even=Transition)
         self.source_list = QListWidget()
@@ -1008,6 +1007,7 @@ class SequenceLinkerUI(QWidget):
         self.content_splitter = QSplitter(Qt.Orientation.Horizontal)
         self.content_splitter.addWidget(file_list_panel)
         self.content_splitter.addWidget(self.preview_tabs)
+        self.content_splitter.setHandleWidth(5)
         self.content_splitter.setSizes([350, 450])
 
         # Export buttons layout
@@ -1048,6 +1048,7 @@ class SequenceLinkerUI(QWidget):
         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.setSizes([250, 750])
         self.main_splitter.setStretchFactor(0, 0)  # Source panel doesn't stretch
         self.main_splitter.setStretchFactor(1, 1)  # Main content stretches