feat: add page attention badge

[saurabhhhcodes]

What changed

• Added a shared attention helper in frontend/main.js that changes the tab title to "Come back to LeadOrbit!" when the page is hidden.
• Draws a small red badge onto the favicon while the tab is in the background, then restores both the title and favicon when the user returns.

Why

• Issue LO-063 [Easy]: Dynamic Page Titles and Favicon Notifications #400 asks for page title and favicon notifications so users can notice background activity more easily.
• The helper lives in the shared app shell, so the behavior works across the app instead of one page at a time.

Validation

• git diff --check
• node --check frontend/main.js

Closes #400

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