Group inventory: add page shell and navigation entry

[prodmodfour]

Goal

Add a visible place for the shared party inventory without wiring persistence yet.

Scope

• Add route constants for /group-inventory or the chosen path.
• Add a primary navigation entry labeled Inventory visible to GM and players.
• Add src/pages/group-inventory.vue with AppNavigation, page title, empty/loading/error states, and a placeholder panel.
• Keep the page static in this ticket; do not call the API yet.

Acceptance criteria

• GM and player nav both show the Inventory link.
• The page renders successfully and has a sensible title/head metadata.
• Guest behavior follows existing route/auth conventions.
• No persistence, save, or transfer behavior is added in this ticket.

Depends on

None.