fix: cross-cutting consistency — DocType, command palette, PWA, meta tags
- Unify DocType to 'doc' | 'sheet' | 'form' | 'slide' | 'diagram' across
workspace.ts, cross-doc-links.ts, and server/index.ts
- Fix countByType using wrong plural keys (docs/sheets/slides → doc/sheet/slide)
- Add command palette (Cmd+K) to sheets, diagrams, slides, and forms editors
- Wire up PWA shortcut actions (?action=new-doc etc) in landing page
- Add 3 missing PWA manifest shortcuts (form, slide, diagram)
- Add missing pages to SW precache, bump cache to v5
- Add theme-color meta and apple-touch-icon to all 6 HTML files
- Add OG meta tags and a11y focus detection to diagrams/slides/forms HTML
- Fix command palette icons and fetchDocuments path/icon maps for all 5 types
- Fix localStorage migration non-null assertions in landing.ts