Forward port fix #8410: copy/cut/paste blocks inside container blocks to …

[nileshgulia1]

…main

Ports the container-level multi-selection and clipboard toolbar from fix-copy-blocks-eea-style (Volto 18) to main (Volto 19).

Changes:

• Container/Edit.jsx: container-local selected/multiSelected state and BlocksToolbar for child blocks.
• Container/EditBlockWrapper.jsx: pass modifier clicks through to child selection instead of forcing single selection.
• Block/Edit.jsx: detect events originating in nested block forms and only activate the container as a single selection; preserve modifier click semantics on the container chrome.
• Form/Form.jsx: use props.uiState.selected when promoting the active block into a multi-selection; avoid null ghost selections.
• BlocksToolbar.jsx + cloneBlocks.ts: read blocksClipboard from the split localStorage keys (blocksClipboard.cut / blocksClipboard.copy) written by persistentReducers, fixing cross-tab sync/remount restore.
• Add unit tests for Block/Edit, Container/Edit, and Form selection.
• Add Cypress tests for copy/cut into Grid.

Adapted for main:

• Kept Volto 19 saveAsDraft wiring in Form.jsx/Form.test.jsx.
• Container/Edit.test.jsx uses main's Loadable.jsx mock path.

Refs #8410

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• I verified there aren't other open pull requests for the same change.
• I followed the guidelines in Contributing to Volto.
• I successfully ran code linting checks on my changes locally.
• I successfully ran unit tests on my changes locally.
• I successfully ran acceptance tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes, either in the Storybook or narrative documentation.
• I included a change log entry in my commits.

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