fix(telegram): restore /start menu and UX copy

[scottishwidow]

Summary

• rename detailed song source label to Джерело (оригінал)
• restore Пропустити button for optional chart key input (keep Скасувати)
• publish /start command on startup and set Telegram chat menu button to Commands
• add implementation note in docs/features/

Changes

• src/handlers/repertoire.py: update format_song source label copy
• src/handlers/charts.py: use skip_cancel_markup for chart-key prompt
• src/bot/application.py: replace command deletion with:
  • set_my_commands([BotCommand("start", "Відкрити головне меню")])
  • set_chat_menu_button(MenuButtonCommands())
• tests updated in:
  • tests/test_handlers.py
  • tests/test_application.py
• docs updated:
  • docs/features/start-command-recovery-and-copy-fixes.md
  • docs/features/README.md

Validation

• UV_CACHE_DIR=.uv-cache uv run pytest tests/test_handlers.py tests/test_application.py ✅ (50 passed)
• UV_CACHE_DIR=.uv-cache uv run ruff check . ✅
• UV_CACHE_DIR=.uv-cache uv run ruff format --check . ✅
• UV_CACHE_DIR=.uv-cache uv run mypy ✅
• UV_CACHE_DIR=.uv-cache uv run pytest ⚠️ hangs in this environment at tests/test_repertoire_backup_service.py::test_export_backup_contains_manifest_and_chart_files (timed out while isolating)

Notes

Telegram does not provide a reliable API to re-show the large first-run "Start" prompt for users who cleared history; exposing /start in Commands menu is the practical recovery path.

[scottishwidow]

Self-review completed.

No blocking findings found in the PR diff. The handler changes are narrowly scoped, use existing reply-keyboard helpers, and the startup change uses set_my_commands plus MenuButtonCommands from the installed python-telegram-bot API.

Residual caveat: targeted tests, ruff, format check, and mypy passed locally. Full pytest did not complete in this environment because it hangs at tests/test_repertoire_backup_service.py::test_export_backup_contains_manifest_and_chart_files, which appears unrelated to this PR's touched files.