Add slot swap controls to dashboard editor

[multiplex55]

Motivation

• Allow users to mark a slot as a swap source and swap slot content (widget, settings, and label) between two slots to streamline dashboard editing workflows.
• Provide a faster UX path to swap with a currently selected_slot and support explicit source/destination swapping via a swap_anchor.
• Ensure swaps respect existing slot validation rules so swaps do not introduce overlaps or out-of-bounds slots.
• Clear any conflict warnings when a swap succeeds so the editor state remains consistent.

Description

• Track a swap source in the editor state by adding swap_anchor: Option<usize> to DashboardEditorDialog and an ensure_swap_anchor helper to keep it valid.
• Add UI controls to each slot header: a Swap button that toggles/consumes the swap_anchor and a Swap with selected action that swaps with selected_slot when present.
• Implement swap_slots(first, second, registry) which swaps widget, settings, and id for the two slots, validates both swapped slots via validate_slot, and rolls back on validation failure while clearing blocked_warning on success.
• Adjust anchor bookkeeping when slots are removed and ensure selected_slot and swap_anchor are kept in sync via ensure_selected_slot and calls to ensure_swap_anchor.

Testing

• No automated tests were run for this change.

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Codex Task