Trail Planner

› In trail-planner/ there is an existing multi-file vanilla JavaScript app; read README.md and the src/ modules first and follow their conventions. Using only Node built-ins and browser APIs (no installs): add filters for difficulty, region, maximum distance, loop only and dogs allowed, plus sorting by rating, distance, elevation gain and estimated time. Keep the filter and sort logic as pure functions in a new src/ module with node:test tests in test/, show the matching count and a clear empty state, keep the selected trail sensible when it is filtered out, run node --test until everything passes, and keep index.html working with relative paths only.