Extend login flow with party selection

Acceptance Criteria
Design Notes

This page is a capture in the next bucket of the product backlog — a pre-sprint idea, not yet pulled into a sprint as a story.

After authentication, look up the user's associated parties from account_parties. If one party, auto-select. If multiple, return the party list to the client for selection. The client displays a party picker widget. After selection, compute the visible party set and bind to the session.

Acceptance Criteria

Login response includes available parties.
Party picker displayed when user has multiple parties.
Selected party stored in session and displayed in application window (title bar shows username @ party_name).
Party context immutable for session lifetime.

Design Notes

Protocol changes

Extend login_response (0x2006) with two fields appended to the wire format:

available_parties: vector<{uuid id, string name}> — empty when 0 or 1 party (auto-selected); populated when ≥2 parties.
selected_party_id: uuid — non-nil only when the server auto-selected exactly one party; nil when the client must choose.

Add two new messages:

select_party_request (0x2015): {uuid party_id}
select_party_response (0x2016): {bool success, string error_message}

Server changes (`handle_login_request`)

The party resolution block (lines 488-520 of accounts_message_handler.cpp) currently always auto-selects parties.front(). Change to:

0 parties → leave session unbound; no change.
1 party → auto-select as now; set selected_party_id in response.
≥2 parties → do NOT bind party_id yet; return available_parties list; leave session unbound until select_party_request arrives.

Add handle_select_party_request: validate chosen party_id is in account's account_parties, bind session->party_id, recompute visible_party_ids, update both in-memory session and DB record.

Client changes

LoginResult gains available_parties and selected_party_id.
ClientManager gains current_party_id_, current_party_name_, and a selectParty(uuid) method.
LoginDialog::onLoginResult gains a new branch after the password-reset check: if available_parties.size() > 1, show PartyPickerDialog (modal, same pattern as ChangePasswordDialog). On reject → disconnect, re-enable form. On accept → call clientManager_->selectParty(id).
Main window title set to username @ party_name after login completes.

Session state machine

Not connected
  → Connected
  → Authenticated  (party-unbound, only when ≥2 parties)
  → Party selected (fully operational)

0 or 1 party: Authenticated → Party selected in one step (no picker shown).