docs: rewrite README to match codebase and range42 org conventions

[t0kubetsu]

Summary

Complete rewrite of the README to accurately reflect the current codebase and follow the documentation conventions established by other range42 repositories. Closes #56.

Changes

Structure (org convention fixes)

• Replace # Table of Contents H1 with # range42-deployer-ui (Pattern A, matching strong range42 repos)
• Add "Part of the range42 platform" anchor link in opener
• Remove copy-paste boilerplate ("What we build" / "Repository Overview") shared verbatim with range42-catalog and range42-api-definitions
• Add ASCII architecture diagram: Operator browser → deployer-ui → backend-api → playbooks → Proxmox
• Add cross-links to range42-backend-api and range42-playbooks

Stale claims corrected

• Remove "Future versions will integrate SQLite WASM" — package is already a production dependency
• Remove "from the UI (coming soon)" — deployment is fully implemented
• Correct storage description: per-project settings use browser cookies, not only localStorage
• Add Japanese (jp) as supported i18n locale alongside en and fr

Missing features documented

• Infrastructure import from live Proxmox (VMs + LXC with real-time status)
• Template & ISO browser
• Git-backed inventory system (multi-repo, read/write via GitHub API)
• Full deployment engine: pause / cancel / resume / retry / skip steps
• VM lifecycle management per role (start, stop, pause, resume, snapshot, revert, delete)
• Real-time Proxmox polling (5s interval)
• Project management dashboard (search, grid/list, node stats, duplication)
• Settings view (/settings route)

New sections added

• Tech stack table
• Project structure tree
• Prerequisites (Node version, backend-api dependency)
• Data & storage table
• Testing commands

Files Changed

File	Change
README.md	Modified (144 insertions, 70 deletions)

Testing

Documentation-only change. Verified all file paths, package versions, locale codes, and npm scripts against the live codebase.

Related Issues

Closes #56