chore: Scaffold web/ monorepo with pnpm workspaces

[jade-codes]

Description

Create the web/ folder structure with Bun workspaces for the SysML visualizer.

Packages

• @syster/diagram-core - Shared rendering, node types, layout
• @syster/viewer - Read-only, embeddable viewer
• @syster/modeller - Full editing with LSP integration

Tasks

• Create web/ folder
• Create root package.json with workspaces field
• Create workspace scripts (build, test, dev)
• Shared tsconfig.base.json
• Shared ESLint config
• Shared Prettier config
• Create package scaffolds (empty src/index.ts)

Acceptance Criteria

• bun install works from web/ root
• bun run build runs (even if empty)