feat: support multi-selection in tangram==0.2.1

[abc8747]

• subscribe to activeEntities instead of activeEntity
• added workspace pyproject.toml for ruff checks and eslint.config.ts for prettier formatting (we might want to merge it with the root pyproject.toml in the future, but keeping it separate for now)

Reproducing locally will require additional steps since v0.2.1 is not yet published on PyPI and npm (awaiting open-aviation/tangram#45).

Assuming traffic, tangram and tangram_exe are co-located in the same directory, first uncomment this in tangram/tangram_landing/pyproject.toml:
https://github.com/abc8747/traffic/blob/fb69d35501efa0e8aeeacc47328f50348278b73a/tangram/tangram_landing/pyproject.toml#L15
and this tangram/pnpm-workspace.yaml:
https://github.com/abc8747/traffic/blob/fb69d35501efa0e8aeeacc47328f50348278b73a/tangram/pnpm-workspace.yaml#L6-L7

Then run

# in 'traffic/tangram/' and 'tangram'
pnpm i
pnpm build
# in 'tangram_exe/'
uv tool install \
  --python 3.13 \
  --with-editable "../tangram/packages/tangram_airports" \
  --with-editable "../tangram/packages/tangram_system" \
  --with-editable "../tangram/packages/tangram_jet1090[history]" \
  --with-editable "../tangram/packages/tangram_history" \
  --with-editable "../traffic/tangram/tangram_landing" \
  --editable "../tangram/packages/tangram_core"
tangram serve --config tangram.toml

ref: open-aviation/tangram#44