Remove work-related terminology by renaming "workspace" to "instance"
across the entire application. This change better reflects that Memos
is a self-hosted tool suitable for personal and non-work use cases.

Breaking Changes:
- API endpoints: /api/v1/workspace/* → /api/v1/instance/*
- gRPC service: WorkspaceService → InstanceService
- Proto types: WorkspaceSetting → InstanceSetting
- Frontend translation keys: workspace-section → instance-section

Backend Changes:
- Renamed proto definitions and regenerated code
- Updated all store layer methods and database drivers
- Renamed service implementations and API handlers
- Updated cache from workspaceSettingCache to instanceSettingCache

Frontend Changes:
- Renamed service client: workspaceServiceClient → instanceServiceClient
- Updated all React components and state management
- Refactored stores: workspace.ts → instance.ts
- Updated all 32 locale translation files

All tests pass and both backend and frontend build successfully.

🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude <noreply@anthropic.com>