feat(frontend): stream live gaze points during calibration

[Avinash-Alapati]

What this adds

• Streams live gaze points from the calibration UI to the backend
• Sends gaze coordinates, phase (training/validation), and timestamps
• Throttled to avoid excessive network traffic

Why this is needed

The backend now supports real-time gaze ingestion and WebSocket broadcasting, but the frontend had no mechanism to emit live gaze data during calibration.

This change provides the minimal frontend hook required to validate the real-time pipeline end-to-end, without affecting existing calibration logic.

Scope

• Minimal, non-breaking change
• No UI changes or observer visualization yet
• Only active during calibration flow

Next steps

• Add observer-side visualization of live gaze points
• Persist gaze data for replay and analysis

[Avinash-Alapati]

This PR complements backend PR #45 (real-time gaze ingestion and streaming).