Normalize share comments response shape

[GChaucer]

Summary

• Normalize GET /share/{token}/comments so the no-target fallback returns a bare array.
• Simplify the single-asset share page comments response type now that the endpoint returns a consistent array shape.
• Add backend regression coverage for asset-share, folder/project-share query asset, and no-target fallback paths.

Closes #72

Testing

• . .venv/bin/activate && pytest apps/api/tests/test_share_comments.py
• . .venv/bin/activate && pytest apps/api/tests/test_share_comments.py apps/api/tests/test_smoke.py
• git diff --check

Notes

• Installed API test dependencies in a local virtualenv.
• Frontend lint was not run because npm ci --ignore-scripts fails due package-lock/package.json drift, and pnpm install --frozen-lockfile --ignore-scripts fails because pnpm-lock.yaml is out of sync with package.json.

[ravirajsinh45]

LGTM — clean implementation of the normalization from #72. Backend → array, frontend simplified, tests cover all three paths (asset-share, folder-share via ?asset_id, no-target empty). Mock pattern matches the rest of the suite.