Group inventory: add transfer UI between party inventory and trainers

[prodmodfour]

Goal

Let users move items between the shared group inventory and trainer inventories from the group inventory page.

Scope

• Add a transfer modal or inline transfer action to group inventory rows.
• Let the user choose an eligible trainer sheet.
• Let the user choose quantity for stackable items.
• Treat equipment as whole-row transfers.
• Call the group-to-trainer and trainer-to-group transfer APIs.
• Update the local group inventory and affected trainer sheet state from authoritative responses.
• For players, show only profile-linked trainer choices.

Acceptance criteria

• GM can transfer items to/from any trainer sheet.
• Player can transfer items only to/from linked trainer sheets.
• Transfer controls handle loading, success, validation errors, and stale revision errors.
• UI does not silently mutate local state before the authoritative response.
• Component/page tests cover at least one successful transfer and one rejected transfer.

Depends on

• Group inventory: add read-only panel using shared inventory components #35
• Group inventory: add atomic group-to-trainer transfer route #38
• Group inventory: add atomic trainer-to-group transfer route #39
• Group inventory: allow player transfers for linked trainer sheets #40