fix(ui): use GET-first flow for imported rooms to avoid invalid PUT

[sanity]

Problem

When a user imports an identity (export → leave room → import), the room is created with a ChatRoomStateV1::default() that has an unsigned configuration. The sync process tries to PUT this invalid state to the contract, which fails with "State verification failed: Invalid signature: signature error" because only the room owner can sign the configuration. Since the PUT fails, the subscription is never established, and the user is stuck in an infinite loop of failed PUT retries with the same invalid default state. All subsequent message UPDATEs also fail.

Diagnostic report UF32XU confirms this: the first error is a PUT failure at 11:53:16, followed by repeated UPDATE failures, all with the same "Invalid signature" error on configuration validation.

Approach

For rooms awaiting subscription, detect "fresh imports" using the existing is_awaiting_initial_sync() method (empty members + empty messages + user is not owner). For these rooms:

1. Send GET instead of PUT to retrieve the real state from the network
2. On GET response, merge the retrieved state into the local room
3. Then PUT the valid state with subscribe: true to register the contract code and establish the subscription
4. Trigger signing key migration after the state is populated

This mirrors the existing invitation flow (which also does GET-first) but for the import path.

Testing

• cargo test -p river-core — all pass
• cargo test -p chat-delegate — all pass
• cargo check for UI compiles cleanly (only pre-existing CSS asset warning)
• No WASM changes — only ui/ code modified, no delegate migration needed

Changes

File	Change
room_synchronizer.rs	Split rooms-to-subscribe into GET-first (imports) vs PUT (normal)
get_response.rs	Handle GET for imported rooms: PUT+subscribe with retrieved state
sync_info.rs	Add get_sync_status() accessor

Closes #188

[AI-assisted - Claude]