v2: multi-layer shift for auto-move

[gorlix]

Context

The v2 auto-move (#7) must keep all layers visually coherent. Shifting only the current layer would misalign content that spans layers 1–3.

Goal

Auto-move shifts content on all layers below the cut line by the same delta (subject to the v1 layer toggle in #1).

Approach

• For each layer (0–3), getElements/operate per layer, applying the same vertical shift.
• Respect layer restrictions (custom layers can't hold titles/links/five-stars — SKILL.md "Layer Restrictions").
• Tie into the layer toggle (chore: add Claude Code skill, MCP config and editor settings #1): "current layer" vs "all layers".

Acceptance criteria

• With "all layers" selected, every layer's content below the line shifts together.
• Layer-restricted element types handled correctly.

References

• Depends on chore(deps): Bump react-test-renderer from 19.0.0 to 19.2.7 #7; related to chore: add Claude Code skill, MCP config and editor settings #1
• SKILL.md "Layer Restrictions"