refactor(handlers): centralize conversation helpers

[scottishwidow]

Summary

• Add handlers.conversation for shared user state access, song ID/callback parsing, cancel filters/fallbacks, and home/remove reply markup helpers.
• Refactor repertoire, chart upload, backup import, and navigation handlers to use the shared helper module without changing callback payloads or user-facing copy.
• Add focused helper coverage and update the button-navigation feature note.

Verification

• uv run pytest tests/test_handlers.py tests/test_navigation.py tests/test_handler_conversation_helpers.py (59 passed, 7 existing PTB warnings)
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• pre-commit during git commit passed ruff, ruff-format, large-file, EOF, whitespace, and mypy hooks

Note: pre-existing untracked docs/plans/ work was left untouched.