refactor: replace lock unwraps with poison-aware helpers

Add SoulseekRs::LockPoisoned variant and a small RwLockExt/MutexExt
helper module exposing read_safe/write_safe/lock_safe that map
PoisonError to that variant. Walk every non-test lock acquisition
across client, peer/server actors, peer registry, listener and the
thread pool: propagate via ? where the function returns Result, log
and degrade to a sensible default (return early, false, None, empty)
where it does not. Client::connect now returns Result so the CLI can
surface the failure instead of panicking.

soulseek-rs-lib/src/error.rs

@@ -18,6 +18,8 @@ pub enum SoulseekRs {
     NotConnected,
     /// Compression/decompression error
     CompressionError(String),
+    /// A lock was poisoned by a panic in another thread
+    LockPoisoned,
 }
 
 impl fmt::Display for SoulseekRs {
@@ -39,6 +41,9 @@ impl fmt::Display for SoulseekRs {
             SoulseekRs::CompressionError(msg) => {
                 write!(f, "Compression error: {}", msg)
             }
+            SoulseekRs::LockPoisoned => {
+                write!(f, "Lock poisoned by panicking thread")
+            }
         }
     }
 }