feat: add AI floating assistant

[Loyal-Young]

What changed

• add a global floating AI assistant to the employee workspace and customer portal
• answer product feature and navigation questions with clickable in-app entry points
• filter suggested entries by the authenticated user's role
• use the configured live AI provider when available, with a deterministic navigation fallback
• add responsive, keyboard-accessible chat interactions and retry handling
• add frontend interaction tests and backend role-scope tests

Why

Users need a lightweight way to find QualiFlow features and entry points without searching through the full navigation. The assistant keeps guidance available while respecting the existing authorization boundaries.

User and developer impact

• employees and customers see an AI assistant button after login
• users can ask where a feature is and navigate directly from the response
• restricted pages are not recommended to unauthorized roles
• no additional configuration is required for navigation fallback; live AI uses the existing AI environment settings

Validation

• frontend: 58 Vitest tests passed
• backend: 159 pytest tests passed
• TypeScript type checking passed
• ESLint passed
• Next.js production build passed
• Ruff checks passed for all changed backend files

Notes

The repository-wide Ruff run still reports four pre-existing import-order findings in migration files unrelated to this change.