Move blocks up/down (reorder siblings)

Summary

Allow reordering blocks among siblings by moving them up or down. This is a basic outliner operation present in Roam web and every outliner tool.

Roam web behavior: Cmd+Shift+Up / Cmd+Shift+Down moves the selected block up or down within its siblings, changing its order value.

TUI adaptation: Use Alt+Up / Alt+Down across all presets (consistent with most editors). Vim preset could also support K / J (shift+k/j) as alternatives.

Implementation hints

Existing pattern to follow

• src/app.rs:1071 — handle_indent() is the closest pattern: it finds the block, computes new parent/order, applies locally, returns WriteAction::MoveBlock
• src/app.rs:1122 — handle_dedent() same pattern
• The new handle_move_up() / handle_move_down() functions should follow this exact structure

API

• src/api/types.rs:111-115 — WriteAction::MoveBlock { block, location } already exists and supports this
• Set location.parent_uid to same parent, location.order to order - 1 (move up) or order + 1 (move down)

Local state update

• Find the block in state.days[].blocks tree
• Swap order values with the sibling above/below
• Reorder the children vec of the parent to reflect new order
• Update selected_block index to follow the moved block

Edge cases

• Block is first child → move up does nothing
• Block is last child → move down does nothing
• After move, the flat index changes — recalculate selected_block to track the block

Keybindings

• src/keys/preset.rs — add new actions: MoveUp, MoveDown (or MoveBlockUp, MoveBlockDown to avoid confusion with cursor movement)
• Map to Alt+Up / Alt+Down in all presets

Undo/redo

• Add UndoEntry::MoveBlock variant or reuse existing one
• Store: block_uid, old_parent_uid, old_order, selected_block

Acceptance criteria

• Alt+Up moves block before its previous sibling
• Alt+Down moves block after its next sibling
• API receives MoveBlock write action
• Selection follows the moved block
• Undo/redo works for move operations
• No-op at boundaries (first/last child)