fix(ux): align bot flow messaging

[scottishwidow]

Summary

• align Ukrainian user-facing copy across search, tags, add song, upload chart, archive, and backup flows
• centralize shared handler copy constants to keep admin rejections, empty states, and next-action prompts consistent
• update docs to reflect button-first UX surface (/start only typed entry/reset) and document arrangement_notes as persisted but currently non-user-facing
• add/update implementation notes in docs/features/button-navigation-cleanup.md for Stage 4 and Stage 5

Testing

• uv run ruff check .
• uv run ruff format --check .
• uv run mypy
• uv run pytest

Commits

• 7adf3c6 fix(ux): align bot flow messaging
• 3363561 docs(features): document existing flow ux cleanup