Use UTC timestamps

[hiroshihorie]

The timestamp() constructor uses now() + UTC.

[Copilot]

Pull Request Overview

This PR updates timestamp generation throughout the codebase to use UTC timestamps instead of local time. The change replaces DateTime.now() with DateTime.timestamp() to ensure consistent UTC-based time handling across the application.

Key changes:

• Replaced all instances of DateTime.now() with DateTime.timestamp() for UTC consistency
• Updated import statements to be more specific by using show clauses
• Modified one instance to reuse an already computed timestamp value instead of creating a new one

Reviewed Changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
web/e2ee.sfi_guard.dart	Updated SIF frame timing calculations to use UTC timestamps
lib/src/support/region_url_provider.dart	Updated region settings cache timing to use UTC timestamps
lib/src/participant/participant.dart	Updated participant speaking and join time tracking to use UTC timestamps
lib/src/participant/local.dart	Updated data stream timestamps and optimized timestamp reuse
lib/src/core/signal_client.dart	Updated ping timestamps to use UTC
lib/src/core/room.dart	Updated transcription and data stream timing to use UTC
lib/src/core/engine.dart	Updated reconnection timing to use UTC

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[Copilot]

This change improves maintainability by reusing the timestamp from info instead of generating a new timestamp with DateTime.timestamp().millisecondsSinceEpoch. This ensures consistency between the timestamp stored in info and the one used in the protocol buffer.