feat: add the official AI Assistant module

[gdarko]

Summary

• adds the free official AI Assistant module with chat, conversation history, and contextual text generation
• provides instance and company settings, with masked API-key handling and company inheritance
• migrates the legacy core AI tables/settings and implements optional data cleanup during uninstall
• uses host-owned authorization, data-reader, and settings contracts instead of app internals
• validates provider URLs to prevent private-network access
• includes reproducible frontend assets, package validation, CI, and the signed marketplace release workflow

Dependencies

• refactor(ai): extract assistant into an official module InvoiceShelf#749 provides the paired host extraction and extension surfaces
• feat: publish AI module contracts modules#6 is merged and released as SDK 3.3.0

Verification

• Composer resolves invoiceshelf/modules 3.3.0 from the published tag
• Pint passes
• PHPUnit: 30 tests, 87 assertions
• module manifest and package validation pass
• ESLint and TypeScript pass
• production build is reproducible with no dist diff