Inventory UI: extract reusable section tabs from trainer inventory panel

[prodmodfour]

Goal

Extract the inventory section tab UI so group inventory and trainer inventory can share the same section navigation.

Scope

• Extract the subtabs/counts portion of TrainerInventoryPanel.vue into a reusable component.
• Continue to use TRAINER_INVENTORY_SECTIONS as the source of section metadata.
• Preserve active-tab behavior and section counts for trainer sheets.
• Keep this as a behavior-preserving refactor.

Acceptance criteria

• Trainer inventory tabs, counts, and active-section switching behave the same as before.
• Tests cover section switching and count rendering if not already covered.
• No group inventory route, persistence, or transfer behavior is added in this ticket.

Depends on

• Inventory UI: extract reusable item table from trainer inventory table #32