Clean up the dashboard slot assignment popup

[iTerminate]

Summary

• Extract the row-aware focus-nav helpers duplicated in light_screen.py/media_player_screen.py into a shared src/hatty/ui/focus_nav.py
• Add ↑/↓ field navigation to DashboardSlotPopup (the "add widget" dialog), matching the control screens' convention; the entity table, selected-entities list, and an open type dropdown keep their own row cursor
• Fix the dialog rendering off-centre/full-width: #dashboard_slot_container's width: auto was defeated by the Footer() composed inside it, so the width is now set explicitly from the existing preview-layout constants

Closes #36

Test plan

• uv run ruff check .
• uv run pyright
• pytest (full suite, 1249 passed)