[AlxcBTW]

What was missing on main

A two-account run showed a guest on Notifications or an empty Home never saw a share play invitation; it only appeared on the party page, and the first invitation expired unseen.

1. The invitation was only a toast (model.toast), with no entry in the Notification Center, unlike a party invitation. Missing.
2. The Friends Home "Share Play" row does not depend on the library, but it required a party to be shown. The launcher Home itself (empty "No games yet" included) has no online rows; its only surface is the bell, which the invitation never lit. Missing.
3. The toast already showed on every page except the open Friends panel. Present, unchanged.

What changed

• New Act::JoinShare(id). The invitation is announced like a party invitation (toast with the invite cue, entry of kind Invitation, text "Lumen invites you to share play"). With the Friends panel open only the entry is kept.
• Accept in Notifications runs the same SharePlayCommand::Answer as the Accept row on the party page and removes the entry. When the invitation is answered or has run out the entry has no button (Dismiss stays). The unread entry lights the bell badge, also on an empty library.
• The Home "Share Play" row shows for an invitation even before the player is in a party.
• No new strings (existing toast text and Accept), so all six languages are covered.
• Tests: toast plus entry plus Accept, panel-open case, answered invitation, Home row without a party.
• DESIGN.md note in the Share play section.

Scenes

notifications-share (Notification Center with the invitation on top) and home-empty-share (empty library, bell with badge).

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