Move dashboard slot preview to a responsive side-by-side pane

[iTerminate]

Summary

• The dashboard slot popup's widget preview was stacked above the entity picker with a fixed height, so on short terminals the popup's content overflowed its 80%-height cap; the code worked around this by hiding the preview outright on the panel/fill entity step, making its visibility depend on widget type and step rather than terminal size.
• The preview now lives in a fixed pane to the right of the popup's main column, shown only when the terminal is wide enough to fit both side by side (preview_fits(), threshold 107 cols); below that it's hidden entirely, and narrow terminals get back the vertical space the stacked preview used to take.
• on_resize re-evaluates and repaints the preview live if the terminal is resized while the popup is open.

Closes #11

Test plan

• pytest tests/unit/test_slot_preview_layout.py — boundary cases for preview_fits
• pytest tests/test_dashboard_slots.py — 16/16 pass, including new narrow/wide acceptance tests
• Full pytest — 1093 passed
• uv run ruff check . — clean
• uv run pyright — 0 errors