Fix integration tests for agents, file sync, and audio. Restore 2026 model versions.

style(chat): optimize markdown preview formatting to be vastly more compact and remove excessive whitespace

Refactor AI Config and Fix Login failure: Migrated services to a structured api directory, updated backend schemas for multiple TTS/STT providers, and improved settings/chat UI coverage.

refactor(frontend): move remaining pages into feature modules

refactor(frontend): move chat UI into feature module and update imports