feat: add Start static workspace and local document library

[CheaperjamRen]

Summary

• add a zero-model-call Start workspace for notes, Todo and local document management
• add document trees, backlinks, attachment semantics, archive/trash recovery and linked Todo receipts
• add Word-like rich Markdown editing with source mode, callouts, math, code, Mermaid and editable GFM tables
• close visual gaps found by Kimi and independent review

Verification

• npm test: 267 files / 3323 tests passed
• typecheck, bundled Skill verification, renderer/main builds passed
• packaged Electron journey passed with zero model calls and 0 renderer errors
• Kimi: 91/100 READY; independent audit: 92/100 READY

Package

• Leemo Setup 0.1.2 built locally with Office Skills included