Refocus app shell tests on frontend contracts

[adrunkhuman]

Summary

• replace brittle raw HTML string checks in test_app_shell.py with parsed element/attribute contract checks
• remove low-value CSS and JavaScript implementation-detail assertions that break on harmless refactors
• add Node-backed runtime tests for frontend sidebar and probe behavior instead of source-string checks
• reduce the full local suite from the #67 baseline of about 25s to 12.54s on this branch

Why

The app-shell tests were over-specifying incidental markup, CSS, and JS source strings. That made small frontend refactors look risky without proving much user-facing behavior. This keeps the important contracts covered while moving behavior assertions into runtime-style frontend tests where possible.

This also addresses the test-suite runtime problem from #67: the suite is now materially faster while preserving the behavior coverage that matters.

Testing

• uv run pytest tests/test_app_shell.py tests/test_app_frontend.py tests/test_probe_frontend.py -q
• uv run ruff check tests/test_app_shell.py tests/test_app_frontend.py tests/test_probe_frontend.py
• uv run ty check tests/test_app_shell.py tests/test_app_frontend.py tests/test_probe_frontend.py
• uv run pytest (166 passed in 12.54s)

Closes #67