Client-side follow-up to server federation (accordserver#38).

The Accord server now speaks peer-to-peer federation, but the vendored SDK (packages/accordkit) has no federation support: no origin field on any model, no qualified-ID (<snowflake>@<domain>) handling, and no endpoint to join a remote-homed space. Nothing in the client can currently consume a federated space.

This issue covers the protocol/SDK plumbing that the Flutter UI work depends on.

Task

• origin field: add the optional origin (home domain) field that the server now returns on users, spaces, channels, roles, messages, and members. Parse and round-trip it on the relevant Accord* models.
• Qualified IDs: tolerate IDs that cross the wire qualified as <snowflake>@<domain> without breaking lenient snowflake/timestamp parsing (server strips @domain for timestamps — mirror that). Remote authors also carry fully-qualified handles.
• Join trigger: expose the authed POST /api/v1/federation/spaces/join call (initiates the join handshake and applies the returned snapshot).
• Remote DMs: surface cross-server DM open against a remote recipient (qualified user id).
• Remote media: ensure emoji/attachment URLs that arrive as absolute home-server CDN URLs are used as-is (not re-pointed at the local CDN).

Pointers

• packages/accordkit/lib/src/models/ (add origin)
• packages/accordkit/lib/src/rest/ (federation join endpoint)
• Snowflake/timestamp parsing in accordkit (qualified-ID tolerance)
• Server reference: src/routes/spaces.rs (join_federated_space), src/federation/mapping.rs (qualified-ID helpers), src/federation/handshake.rs (snapshot shape)

Acceptance

• Accord* models expose origin and survive a round-trip where IDs are qualified.
• A client can call the federated-space join endpoint and hydrate the returned snapshot (space, channels, roles, members, recent messages).
• Snowflake-derived timestamps still parse for qualified IDs.