Add Nova theme port

[ALI-OUALA]

Fixes #143.

This adds a Nova port for Aura Theme:

• Adds a Nova extension manifest under packages/nova/Aura Theme.novaextension.
• Adds four Nova theme variants: Aura Dark, Aura Dark (Soft Text), Aura Soft Dark, and Aura Soft Dark (Soft Text).
• Adds source templates and build integration under src/ports/nova.
• Adds Nova install instructions in packages/nova/README.md.

Verification:

• yarn aura build only nova
• node -e "JSON.parse(require('fs').readFileSync('packages/nova/Aura Theme.novaextension/extension.json','utf8')); console.log('manifest json ok')"
• rg "\\{\\{" packages\\nova
• git diff --check
• yarn test tests/unit/utils/to-port-name.spec.ts