fix: dataset panel empty state non-responsive when splitter resized

[Copilot]

The "💡 Select a dataset to view its alignments below" helper label and the "No datasets registered" empty state label were both growing and shrinking as the splitter was dragged, due to Qt distributing extra vertical space evenly across all Preferred-policy widgets when stretch factors are all 0.

Root Cause

In _create_list_section, the outer QVBoxLayout held button_container, helper_label, and list_container all at the default stretch factor of 0. Qt's layout algorithm distributes leftover space evenly between them, causing the blue hint box to visibly resize.

Fix

# helper_label: locked to its natural (text + padding) height
helper_label.setSizePolicy(QSizePolicy.Policy.Expanding, QSizePolicy.Policy.Fixed)

# list_container absorbs all extra vertical space — helper_label no longer competes
layout.addWidget(list_container, 1)  # stretch=1

# empty state label: same treatment
self.empty_label.setSizePolicy(QSizePolicy.Policy.Expanding, QSizePolicy.Policy.Fixed)

• list_container gets stretch factor 1 → it owns all extra space; helper_label stays compact and consistent
• Both labels get Fixed vertical size policy → height is strictly their sizeHint() regardless of panel size

Release Branch PR

Target Release:

Changes

• Fixed helper_label ("Select a dataset…") resizing when splitter is dragged by setting vertical QSizePolicy to Fixed
• Fixed empty_label ("No datasets registered") expanding to fill the panel by setting vertical QSizePolicy to Fixed
• Gave list_container stretch factor 1 in the outer layout so it absorbs all extra vertical space, keeping the hint label at a consistent, smaller height
• Added QSizePolicy to PyQt6 widget imports

Testing

• Test testable logic
• Edge cases addressed

Testing Notes:

Verified with ruff lint check — no issues. Existing GUI test structure confirmed empty_label and helper_label attributes remain accessible and behave as before (hide()/show() paths unchanged).

Documentation

• README updated (if applicable)
• Code comments added/updated
• User-facing documentation updated (if applicable)

Documentation Notes:

No documentation changes required — purely a layout sizing fix.

Checklist

• Code follows project conventions
• No breaking changes (or documented if intentional)
• All CI checks pass