v0.5.2 — Windows installer fixes + output folder

v0.5.2 — Windows installer fixes + output folder

Fixes

• Windows one-liner installer (irm ... | iex) now runs:
  • removed a [ValidateSet] default mismatch on -Backend that aborted the piped
    script (ValidateSetFailure).
  • writes config.json as UTF-8 without a BOM, and the app now tolerates a BOM
    anyway. A BOM'd config previously made Trellis Studio show backend "unknown" and
    an empty models directory despite a good install.
• Windows desktop app is packaged as the NSIS setup .exe Tauri actually builds
  (fixes the missing Windows app installer asset).

New

• Trellis Studio — output folder. Generated GLBs now auto-save to a configurable
  folder (default …/trellis-studio/output), with a native Browse… picker and an
  Open folder button (handy since AppData\Local is awkward to reach in Explorer).

Install / usage: docs/getting-started.md.

Server bundles (trellis-<backend>-<os>-x64) and the app installers
(trellis-studio-*) attach below once the release CI finishes building.