Group inventory: enable GM direct editing and save on the inventory page

[prodmodfour]

Goal

Let GM users edit the shared inventory directly through the group inventory page.

Scope

• Enable row add/remove/edit controls only for GM users.
• Add money editing for GM users.
• POST full-document saves to the revision-checked save API.
• Show saving, saved, conflict, and generic error states.
• On stale save conflict, reload or offer a clear refresh path instead of overwriting.

Acceptance criteria

• GM can add, edit, and remove item rows and money, then save.
• Player users still see read-only inventory controls.
• Save uses expectedRevision and updates the local document to the authoritative response.
• Stale revision behavior is tested at the composable/page level where practical.

Depends on

• Group inventory: add GM-only revision-checked save API route #31
• Group inventory: add read-only panel using shared inventory components #35