fix: fetch server configuration on client initialization for invite_only servers

[riscdanger]

When invite_only: true is set on a Stoat server, the registration form in the web frontend never displays the "Invite Code" input field, making it impossible to register on invite-only instances via the UI.

note: I'm not a typescript dev and this was written by AI.

Thanks :)

Root Cause

The Client constructor in javascript-client-sdk/src/Client.ts initializes configured to true whenever a default configuration is passed in (which Controller.ts always does). The #fetchConfiguration() method then skips fetching the real server config because this.configuration is already truthy.

As a result, features.invite_only was always false (the hardcoded default), so FlowCreate.tsx's <Show when={isInviteOnly()}> never rendered the invite field.

Changes

javascript-client-sdk/src/Client.ts

• Add #configurationFetched flag to track whether the server config has been fetched
• Initialize configured signal to false instead of inferring from whether a default config was passed
• #fetchConfiguration() now always fetches the real server config on first call, then caches

The frontend code in FlowCreate.tsx and Form.tsx was already correct — it just never received the real server config to read invite_only from.

Testing

• Start a Stoat instance with invite_only: true in Revolt.toml
• Visit the web frontend registration page — the "Invite Code" field should now appear
• Submit with a valid invite code — registration should succeed
• Non-invite-only instances should continue to work as before (no invite field shown)