Add unmanaged storage container support

[aurthurm]

Description of the issue/feature this PR addresses

Linked issue: #68

This PR implements unmanaged sample container support in senaite.storage.

StorageSamplesContainer previously assumed all sample containers were grid-managed and required fixed row/column positions. This PR adds support for unmanaged containers that store samples without slot-level position tracking,
while preserving the existing managed behavior.

The PR also updates the related store/list/viewlet flows so unmanaged containers behave correctly in the UI and backend.

Current behavior before PR

• StorageSamplesContainer always behaves as a managed grid container
• storing samples requires selecting a position
• sample listings inside a samples container always show a position column
• stored-sample location viewlets always display a position
• unmanaged-like containers cannot be modeled properly
• editing a container that has unmanaged semantics can trigger row/column validation errors after samples have already been added

Desired behavior after PR is merged

• StorageSamplesContainer supports both managed and unmanaged modes
• managed containers keep the existing row/column position behavior
• unmanaged containers allow storing samples without fixed positions
• unmanaged containers can optionally enforce a physical_capacity
• container listings and stored-sample displays no longer show meaningless positions for unmanaged containers
• the edit form shows the relevant fields depending on mode:
  • managed: rows and columns
  • unmanaged: physical_capacity
• managed capacity stays aligned with rows * columns
• unmanaged containers no longer fail managed-grid minimum-size validation when edited after samples have been stored

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I confirm I have tested this PR thoroughly and coded it according to [PEP8][1]
and [Plone's Python styleguide][2] standards.