feat: add page visibility favicon notifications

[saurabhhhcodes]

What changed

• Added shared page-visibility handling in frontend/main.js so LeadOrbit switches to a "Come back to LeadOrbit!" title when the tab is hidden.
• Added a favicon badge renderer that draws a small red notification dot on the existing icon.
• Hooked the CSV import success path to trigger the notification helper when an import completes while the tab is hidden.

Why

• Users who leave the app in another tab need a lightweight signal that background work finished.
• A title change plus favicon badge is less intrusive than system notifications and works well across the app shell.

Validation

• node --check frontend/main.js
• node --check /tmp/leads-notify-script.mjs
• git diff --check

Closes #400

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