emrg: remove unused imports (traceback in app.py, Span in scrollback.py)

[argszero]

Changes

• app.py: Remove unused import traceback — traceback module never referenced in the file
• scrollback.py: Remove unused Span from import — only Line is used in this module

Verification

uv run pytest tests/ -v # 238 passed
uv run python -c "from emrg.client.app import run_client" # OK
uv run python -m emrg --help # OK

[argszero]

✅ LGTM — cycle #5

[argszero]

✅ LGTM — cycle #6

[argszero]

✅ LGTM — cycle #7