Add a brute-force prime algorithm option #87

Follow-up to #34. PR #85 added an algorithm selector with Eratosthenes and linear sieves and an explanation card for each.

#34 also suggested bringing back a brute-force (trial division) implementation as a selectable option. It would show how much slower a naive approach is.

Tasks

• Add a trial division command in src-tauri/src/main.rs and an option in the algorithm selector.
• Add its explanation to the algorithm card, including that it is much slower for large limits.
• Decide whether to cap its input limit to keep the app responsive.
• Cover it in the Rust tests and decide whether it needs a performance baseline.