Add Split H/V controls and preview tooltips

[multiplex55]

Motivation

• Provide quick split operations in the dashboard editor to let users subdivide an existing slot horizontally or vertically.
• Avoid label overlap in the preview by reducing visible text and surfacing full slot info via hover tooltips.
• Ensure splits are safe by reusing existing validation and occupancy checks and rolling back on conflicts.
• Make the preview more usable by increasing the rendered size of cells.

Description

• Added a SplitDirection enum and two UI buttons Split H and Split V next to Collapse all, which call a new method split_selected_slot.
• Implemented split_selected_slot to split the selected slot into two (top/left keeps existing widget/settings, the other half is a new slot created with SlotConfig::with_widget("weather_site", ...) so settings is initialized), compute half/half spans, and use clamp_slot, occupancy_map, and validate_slot to detect conflicts and roll back on error.
• Increased preview cell size from 40px to 60px via cell_size and removed the full in-slot text in favor of per-slot hover tooltips using ui.interact(...).on_hover_text(...) that show id/widget and position/span.
• Kept existing widget/state for the original slot, used the same default widget for the new slot, and surface warnings via blocked_warning when operations are invalid.

Testing

• Ran cargo fmt to format changes, which completed successfully.
• No unit/integration test suite was executed as part of this change.

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Codex Task