fix: send room deltas instead of full state to Freenet API

[netsirius]

Summary

• Room sync now computes a delta from the last synced baseline and sends UpdateData::Delta instead of serializing the entire ChatRoomStateV1 on every change
• Falls back to UpdateData::State (full state) only on first sync when no baseline exists
• Skips the network send when delta computation returns None, updating last_synced_state to prevent re-queuing
• Extracted pure compute_update_data() function with 4 unit tests

Closes #17

What changed

ui/src/components/app/sync_info.rs

• needs_to_send_update() now returns (ChatRoomStateV1, Option<ChatRoomStateV1>) per room — the current state paired with the last synced baseline

ui/src/components/app/freenet_api/room_synchronizer.rs

• New compute_update_data(state, baseline, params) -> Option<UpdateData> — pure function that computes a delta via ComposableState::summarize() + ComposableState::delta(), or falls back to full state when no baseline exists
• process_rooms() calls compute_update_data() and sends the result
• When delta is None, updates last_synced_state so the room isn't re-queued

Why this is safe

• The room contract already handles UpdateData::Delta in update_state() — deserializes ChatRoomStateV1Delta and calls apply_delta()
• All sub-components are CRDT-like: idempotent, deduplicated by ID, deterministically ordered
• The #[composable] macro enforces field application order (config -> bans -> members -> member_info -> secrets -> messages)
• Freenet nodes compute per-subscriber deltas via get_state_delta() + summaries — this change only affects the client-to-node path
• ensure_members_for_message_authors() in the contract injects missing member entries into deltas, preventing rejection of messages from pruned members

Tests

4 unit tests on compute_update_data():

• no_baseline_returns_full_state — first sync sends UpdateData::State
• identical_states_returns_none — no changes produces None
• changed_state_returns_delta — new message produces UpdateData::Delta
• delta_is_smaller_than_full_state — delta bytes < full state bytes