Improve onboarding, session safety, and chart layout

[lookfirst]

Summary

• Add a first-time welcome screen with open-source, privacy, and TCX export guidance.
• Organize dashboard keyboard help and clarify connection controls.
• Move chart labels outside the plots, contain each series, and use solid top and bottom boundaries.
• Protect recorded active sessions from accidental refresh or window closure.
• Document the task-branch and pull-request delivery workflow.

Why

These changes make the initial experience clearer, prevent chart series from overlapping their labels or neighboring plots, and reduce the chance of losing an active ride through an accidental page exit.

Validation

• bun run ci
• 64 tests passing
• Biome, TypeScript, and production build passing