fix: import rand::RngExt so pick compiles after rand 0.10 bump

[andrew-tawfeek]

main currently fails to compile. The rand 0.10 upgrade in #220 moved random_range off the Rng trait and into the new RngExt trait, but src/cmd/pick.rs still imports rand::Rng:

error[E0599]: no method named `random_range` found for struct `ThreadRng` in the current scope
   --> src/cmd/pick.rs:89:53
    |
 89 |     let problem = &problems[rand::rng().random_range(0..problems.len())];
    |                                         ^^^^^^^^^^^^
    |
help: trait `RngExt` which provides `random_range` is implemented but not in scope; perhaps you want to import it
    |
  2 + use rand::RngExt;

cargo build --release fails on a clean checkout of main.

This is fixed by importing rand::RngExt instead of rand::Rng in the pick handler — that's the trait now providing random_range in rand 0.10. It's verified by checking cargo build --release succeeds, and leetcode pick (random pick) works as before.