[client] Add timeouts to actor state::select / state::mutate calls

[intendednull]

Parent: #108

Problem

Every willow_actor::state::select(...).await and willow_actor::state::mutate(...).await call in crates/client/src/ is awaited without a timeout. If the target StateActor hangs, deadlocks, or is slow-blocked by a large mutation, the calling task (and by transitivity any UI reactive chain) hangs forever.

Fix

1. Add a small helper in crates/client/src/util.rs:

   use std::time::Duration;
   
   pub const ACTOR_CALL_TIMEOUT: Duration = Duration::from_secs(5);
   
   pub async fn with_timeout<T, F>(label: &'static str, f: F) -> Result<T, ClientError>
   where
       F: std::future::Future<Output = T>,
   {
       #[cfg(not(target_arch = "wasm32"))]
       {
           tokio::time::timeout(ACTOR_CALL_TIMEOUT, f)
               .await
               .map_err(|_| ClientError::ActorTimeout(label))
       }
       #[cfg(target_arch = "wasm32")]
       {
           // wasm doesn't have tokio timers; use wasm-timer or gloo_timers.
           // ...
       }
   }

2. Wrap all willow_actor::state::select/mutate calls in with_timeout("context_label", ...).

3. Add ClientError::ActorTimeout(&'static str) variant.

4. On timeout, log the label and return the error to the caller so the UI can retry or show an error state.

Test

• Unit test: register a fake actor that sleeps longer than ACTOR_CALL_TIMEOUT, call a client method that uses it, assert ClientError::ActorTimeout is returned within ~6 seconds.
• Regression: existing happy-path tests should continue passing without modification.

Scope

Don't block on this for every call at once. Start with the hot paths in mutations.rs, accessors.rs, and joining.rs. File a follow-up for the long tail.

Out of scope

• Switching to a different actor system.
• Backpressure / mailbox bounds on actors (separate issue).

[intendednull]

Already implemented in commit f7ebf1e (merged via #146): added util::with_timeout + ClientError::ActorTimeout and wrapped hot-path willow_actor::state::select/mutate calls in mutations.rs / accessors.rs / joining.rs. Closing as resolved — open a follow-up if additional call sites still need coverage.