feat: add percentage-based certificate expiry notifications

[AnuragEkkati]

Summary

In this pull request, the following changes are made:

• Add support for lifetime percentage thresholds in addition to days
• Calculate percentRemaining for certificates based on total lifetime
• Support dashboard tag colors to use 25% threshold. Fallback 7 days
• Provide option in UI settings to configure percentage thresholds (1-100%)
• Gracefully handle certificates without validFrom (Globalping monitors)

This feature fixes #6821

• Relates to Support for shortlived certificates #6821
• Resolves Support for shortlived certificates #6821

Please follow this checklist to avoid unnecessary back and forth (click to expand)

• ⚠️ If there are Breaking change (a fix or feature that alters existing functionality in a way that could cause issues) I have called them out
• 🧠 I have disclosed any use of LLMs/AI in this contribution and reviewed all generated content.
  I understand that I am responsible for and able to explain every line of code I submit.
• 🔍 Any UI changes adhere to visual style of this project.
• 🛠️ I have self-reviewed and self-tested my code to ensure it works as expected.
• 📝 I have commented my code, especially in hard-to-understand areas (e.g., using JSDoc for methods).
• 🤖 I added or updated automated tests where appropriate.
• 📄 Documentation updates are included (if applicable).
• 🧰 Dependency updates are listed and explained.
• ⚠️ CI passes and is green.

AI Usage

Used Cursor AI IDE to review the changes and highlight corner conditions to be explored.

Screenshots for Visual Changes

Notifications Tab:

Webhook Notification:

Status Page :
Tested by configuring 95% of cert validity if it turns red ( reverted to 25% in code )

Stdout Logs:

[ManInDark]

        link.totalDays = dayjs.utc(link.validTo).diff(dayjs.utc(link.validFrom), "day"); 
         link.percentRemaining = link.totalDays > 0 ? Math.round((link.daysRemaining / link.totalDays) * 100) : 0;

If I understand this correctly, you first convert the total and remaining time to a daycount and then calculate the percentage.

I belive this could cause issues with extremely short lived certificates. For example step-ca recommends validity periods of only one day. If I understand the code correctly it would show 100% and then immediately 0%.

Though If the library supports floating point days, this isn't an issue.

[AnuragEkkati]

@CommanderStorm can you please review or assign reviewer for this when you find time.

Thanks

[AnuragEkkati]

@CommanderStorm can you please assign some reviewer to this PR when you find time.

Thanks