Bind online session browser to canvas, drop IMGUI

- OnlineSessionManager: remove OnGUI/IMGUI state; now a logic-only layer
  exposing Host/Refresh/Join/Disconnect plus Status/IsBusy/IsConnected/
  InSession/Sessions and Changed/SessionsChanged events
- OnlineSessionBrowserUI (new): drives the Servers canvas — 4 buttons,
  session list from Entry Template clones (name + players), row selection,
  state-driven interactability, status footer
- Main scene: attach binder, add status footer line, wire 8 refs; dev
  bootstrap panel disabled
- Online Session Manager.prefab (new)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Assets/Prefabs/Online Session Manager.prefab.meta

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