Implement incremental index updates for drag-and-drop reordering

[clFaster]

Goal

Implement incremental index updates for drag-and-drop reordering

Type

Technical

Effort

M

Impact

Medium

Evidence

set_boards_index clears and rebuilds index tables in src-tauri/src/commands/boards.rs, src-tauri/src/commands/boards.rs

First Implementation Step

Add move operations (move_board, move_folder) that update affected rows only