fix: a panic on one thread no longer takes the whole app down

Every shared lock was std::sync::Mutex with lock().unwrap() (~150 sites),
so a panic in any session's reader or callback poisoned the lock and the
next UI frame panicked too, dropping every open session. Locks are now
parking_lot::Mutex (no poisoning; already in the dependency tree), and a
panic hook reports worker panics as an error toast instead of the session
silently stopping (F3, F4).

rust/Cargo.lock

@@ -2806,6 +2806,7 @@ dependencies = [
  "egui-phosphor",
  "image",
  "nexus-core",
+ "parking_lot",
  "regex",
  "rfd",
  "serde",
@@ -2827,6 +2828,7 @@ dependencies = [
  "image",
  "keyring",
  "libc",
+ "parking_lot",
  "portable-pty",
  "rand 0.8.8",
  "rusqlite",