[codex] Fix SuperSplat loading external PLY files

[JoseEliel]

Summary

• allow the SuperSplat data server to serve an explicitly selected local PLY folder outside the app project directory
• stop the SuperSplat viewer if the data server fails to start, avoiding a broken viewer URL
• URL-encode the load= parameter before opening SuperSplat
• enable address reuse for the local data server socket

Root cause

The data server rejected user output paths outside the CorbeauSplat project root with Dossier de données hors du répertoire du projet. Brush exports commonly live in an external results folder, so the viewer could launch but fail to fetch the selected PLY.

The server still binds to 127.0.0.1 and only serves the exact folder selected by the user.

Validation

• python3 -m py_compile app/core/superplat_engine.py app/gui/tabs/superplat_tab.py
• git diff --check
• Started the data server against /Users/eliel/Dropbox/Fun Codes/3Dfun/results/Brucegard/checkpoints and confirmed Brucegard_export_50000.ply returns 200 OK via HTTP HEAD.