disable unnecessary collapsing behaviour in task panel (vibe-kanban) (#1061)

* Fixed! The issue was in [TasksLayout.tsx](file:///private/var/folders/5q/5vgq75y92dz0k7n62z93299r0000gn/T/vibe-kanban/worktrees/a762-task-drag-limits/frontend/src/components/layout/TasksLayout.tsx#L238). The "right" panel (task panel) had `collapsible` set to `true`, allowing users to drag it all the way to hide it. Changed it to `collapsible={false}` so the task panel can be resized but never fully closed when it's the only panel open.

* Done! Also disabled collapsing for the aux panel (preview/diff). Now users can resize both panels but can't accidentally close them by dragging.

frontend/src/components/layout/TasksLayout.tsx

@@ -141,8 +141,7 @@ function RightWorkArea({
               order={2}
               defaultSize={innerSizes[1]}
               minSize={MIN_PANEL_SIZE}
-              collapsible
-              collapsedSize={0}
+              collapsible={false}
               className="min-w-0 min-h-0 overflow-hidden"
               role="region"
               aria-label={mode === 'preview' ? 'Preview' : 'Diffs'}
@@ -235,8 +234,7 @@ function DesktopSimple({
         order={2}
         defaultSize={outerSizes[1]}
         minSize={MIN_PANEL_SIZE}
-        collapsible
-        collapsedSize={0}
+        collapsible={false}
         className="min-w-0 min-h-0 overflow-hidden"
       >
         <RightWorkArea