feat: surface blank DOCX and Markdown creation

[aa1386]

Summary

• add visible Blank document and Blank Markdown cards to the existing Docs home
• route both cards through the existing DOCX/template creation handlers
• clear stale document, recovery, error, title, and desktop file-binding state
• prevent a late template/open operation from replacing a newer creation choice
• keep the cards keyboard-accessible through the existing button-card pattern

Verification

• bun test ./examples/vite/src/Home.test.tsx ./examples/vite/src/App.new-document.test.tsx (4 passed)
• bun run build:demo passed in the implementation checkout
• clean latest-upstream worktree build reached application bundling but its reused dependency tree predates the declared y-indexeddb lock entry; no dependency install was performed

Scope

• existing Open file and non-blank template flows remain app-owned
• no duplicate creation pipeline or editor extraction
• no version change in this PR; versioning is deferred until approval