fix: import rand::RngExt for random_range in pick

The rand 0.10 bump (#220) moved `random_range` off the `Rng` trait into
the new `RngExt` trait, so `src/cmd/pick.rs` no longer compiles on `main`:

    error[E0599]: no method named `random_range` found for struct `ThreadRng`
    help: trait `RngExt` which provides `random_range` is implemented but
          not in scope; perhaps you want to import it

Import `RngExt` instead of `Rng` in the `pick` handler.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: andrew-tawfeek

src/cmd/pick.rs

@@ -44,7 +44,7 @@ impl PickArgs {
     /// `pick` handler
     pub async fn run(&self) -> Result<(), Error> {
         use crate::cache::Cache;
-        use rand::Rng;
+        use rand::RngExt;
 
         let cache = Cache::new()?;
         let mut problems = cache.get_problems()?;