fix(receiver): allow display sleep before first session

[Betafer]

Summary

Fix the Receiver display-sleep regression reported again in #132 on TargetBridge 3.5.

SDL starts with screen-saver inhibition enabled after the display is created. A Receiver waiting for its first client never reaches close_client(), so the inhibition remains active indefinitely. This change explicitly enables normal display sleep immediately after display initialization; the existing accept path still disables it for an active session and close_client() re-enables it afterward.

Scope

• one production file
• no protocol or preference changes
• preserves the existing active-session behavior

Verification

• rebased onto maint-3.5 at v3.5.1-rc.3
• Receiver make test: 67 parser checks, 562 input-queue checks, 13 receiver-profile checks; all pass
• full Receiver build succeeds
• verified on a 2017 Intel iMac running macOS Ventura: while the Receiver is waiting, PreventUserIdleDisplaySleep returns to 0

Closes #132.