Menu: pick the run first and derive its route from it

[vanlueckn]

Summary

• Restructures the main-menu drive flow: the run (scenario, operating-day service, or free run) is now the first step, and the route is derived from it instead of being asked for beforehand. A scenario names its line, a service names the line of its plan.
• Only steps the run leaves open are walked: the line page where the run names none (the free run), the vehicle page where it brings no consists and no vehicle:, and date/weather setup for a service. A prepared scenario that brings its own route and train is one question; the free run is three.
• Adds an Open/flow model in crates/app/src/menu.rs (run_of, Open::flow, next_step, advance) and reworks Page::back so Esc follows the run's actual step chain instead of a fixed page chain.
• The step rail is now dynamic: it shows as many steps as the run under the cursor leaves open, so it never promises a step that will not be walked. Answers are stored per page slot and cleared behind a newly answered step.
• world::build lets the run's own line beat the menu's route, with only --line above it. Runs whose route no installed mod provides are not offered at all.
• Renames the old Page::Scenario/Page::Run pages to Page::Run/Page::Setup; --menu scenario still resolves to the run picker for compatibility, and --menu setup replaces --menu run for the service setup page.
• Updates i18n keys in both en and de locales, and refreshes README.md, STATUS.md and the screenshot skill docs to describe the new flow and page names.

Testing

• Run cargo test --workspace (baseline: 1136 tests green) — includes the i18n parity test over the en/de locale files.
• Run cargo clippy --workspace -- -D warnings and cargo fmt --check.
• Manual: open the menu and confirm Drive starts on the run picker; a prepared scenario goes straight to driving, the free run still walks route → vehicle.
• Manual: pick an operating-day service and confirm the date/weather setup page follows, then the run starts two minutes before departure under the service's route.
• Manual: verify Esc walks back through the run's actual steps and leaves at the title screen from the first step.
• Manual: toggle a mod off in the mod manager and confirm runs on its routes disappear from the run list.
• Manual: check --menu run, --menu scenario, --menu setup and --menu line still render the intended pages.
• Screenshot review of the new step rail on the run picker, route and setup pages.