Guard background messaging and detect startup paths

[geauxtigers]

Summary

This PR adds two defensive guards in the background scripts:

• avoid messaging errors when there is no active content-script receiver
• avoid startup-time detect errors before origin state is initialized

Changes

• in background/messages.js, notifyContent() now:
  • returns early when there is no active tab
  • ignores chrome.runtime.lastError when a tab has no receiving content script
• in background/detect.js, detect() now returns early when the URL or state.origins is not yet available

Why

While testing locally, the extension could emit background errors in normal browsing cases:

• "Could not establish connection. Receiving end does not exist."
• errors caused by reading state.origins before initialization completed

These guards make those paths no-op safely instead of surfacing runtime noise.

Scope

This PR intentionally does not include the copy-button feature work.