feat: refactor frontend navigation and app shell

[arthurpanhku]

Description | 描述

Refactors the frontend app shell and assessment workflow around clearer user tasks and responsive interaction patterns.

• Centralizes navigation metadata and groups destinations by workflow
• Adds a responsive sidebar/drawer, route context, health and governance status
• Adds an accessible command palette with search and Cmd/Ctrl+K
• Moves assessment submission into a focused, URL-addressable dialog
• Refreshes shared cards, controls, spacing, focus states, and dark theme tokens
• Adds layout and command-palette tests

Type of Change | 变更类型

• Bug fix
• New feature
• Documentation update
• Refactor / chore

How to Verify | 如何验证

• Run .venv/bin/pytest
• Run npm run check --prefix frontend
• Run npm run build --prefix frontend
• Run npm audit --omit=dev --prefix frontend
• Open /console/assessments at desktop and mobile widths
• Open New assessment and verify the human-review policy is explicit
• Use Cmd/Ctrl+K, search for mcp, and open Agent gateway

Checklist | 检查项

• Tests pass locally (124 backend and 8 frontend tests)
• Frontend checks pass
• Generated contracts are current
• Security boundaries and denied paths are covered where relevant
• Docs/README updated if needed (not required for this UI-only change)
• No secrets or sensitive data in the diff