#334 Link reports to original LeetCode problems

The completed report now shows an Original problem link directly below its title. The link uses the published problem map to open the corresponding LeetCode page.

The same reference appears in the immediate report, Past attempts, replay, and downloaded Markdown. The lobby loads the map only when a saved report is opened or exported. Unknown or malformed mappings leave the line hidden, and the active interview view is unchanged.

Checks: 138 lobby browser tests, 70 render tests, and 8 history tests passed. Prettier 3.9.9 and ESLint 10.12.0 passed for the changed JavaScript files; the local app was manually tested.

Closes #152