whatif: relax MC-paths step from 500 to 100

step=500 with min=100 made valid values 100, 600, 1100, … so typing
a round number like 500 or 1000 tripped browser validation
("Please enter a valid value. The two nearest valid values are 100
and 600."). Found via headless click-through against the deployed
instance — the Run-simulation submit was silently rejected.

step=100 keeps a sensible up/down increment while accepting any
multiple of 100 (100, 200, …, 5000, …, 50000).

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/pages/WhatIf.tsx

@@ -165,7 +165,7 @@ export function WhatIf() {
               onChange={(v) => update('n_paths', v)}
               min={100}
               max={50000}
-              step={500}
+              step={100}
             />
           </Field>