fix(invite-card): show 'banned from this room' state instead of enabled Accept

[sanity]

Follow-up from PR #278 skeptical review (Important #2).

Problem

already_member = target_room_data.is_some_and(|rd| rd.can_participate().is_ok()). can_participate() returns Err(UserBanned) for banned users, so already_member = false — the Accept button shows the normal "Accept invitation" label. Clicking it tries present_invitation which proceeds to the contract layer, where the join deterministically fails.

Suggested fix

Distinguish ban from not-a-member in the card's button state:

let card_state = if let Some(rd) = target_room_data {
    match rd.can_participate() {
        Ok(_) => CardState::AlreadyMember,
        Err(CanParticipateError::UserBanned) => CardState::Banned,
        Err(_) => CardState::Joinable,
    }
} else {
    CardState::Joinable
};

Render Banned as a disabled button reading "You're banned from this room" with a destructive-tinted color.

Severity

UX nit, not a correctness bug. The contract correctly rejects the join attempt; this is just about giving the user accurate information up-front.

[AI-assisted - Claude]