Docs pass: user guide still describes browser-era behavior

[rebaserHEAD]

docs/user-guide.md (and possibly the other docs/ pages) predates the desktop-first shift in places. Known example: the prefab panel''s folder-open documented as "uses File System Access API, Chrome/Edge only" (line ~310), which reads as product behavior when it''s really the dev-browser fallback; #45 changes that story entirely.

• Sweep docs/ for browser-era instructions and reframe desktop-first with browser-dev as the aside.
• Best done alongside or after Desktop: prefab discovery only works against the dev server #45 (prefab storage) and Save vs Save As: Ctrl+S should save in place once a file has a path #49 (Save vs Save As) since both change user-facing flows.
• Relates to Decide a user-facing documentation system and format #24 (docs system decision).