feat: Idle-aware smart test scheduling using chrome.idle API

[ajithakdev]

Summary

Enhance the scheduled test system to be "idle-aware" — never run bandwidth-intensive tests while the user is actively working, on a video call, or gaming.

Current state

Scheduled tests use chrome.alarms with fixed intervals (configurable in options). Tests fire regardless of user activity.

Proposed improvement

• Use chrome.idle.queryState() before running scheduled tests
• Only run full bandwidth tests when state is 'idle' (user hasn't interacted for 60+ seconds)
• If user is 'active', defer the test and retry after 5 minutes
• If user is 'locked', still run the test (screen locked = not actively using bandwidth)

Why

• Prevents scheduled tests from disrupting active Zoom calls, gaming sessions, or large downloads
• Makes background monitoring truly invisible and non-intrusive
• Addresses the Dev/v2025.2.0 #1 complaint about continuous monitoring tools: "It slows down my browser"

Implementation

• Add chrome.idle permission to manifest
• Wrap runFullTest() call in alarm listener with idle check
• Add configurable idle threshold in options (default: 60 seconds)