Fix all 98 ruff lint errors across codebase
- Remove unused imports (app.py, stego_routes.py, killswitch.py, etc.)
- Sort import blocks (I001)
- Add missing os import in stego_routes.py (F821)
- Rename shadowed Click commands to avoid F811 (status→chain_status, show→chain_show)
- Rename uppercase locals R→earth_r, _HAS_QRCODE_READ→_has_qrcode_read (N806)
- Suppress false-positive F821 for get_username (closure scope)
- Use datetime.UTC alias (UP017)
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
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