feat: Added support for extra links and rewrote the cats template

[tarampampam]

Summary

Implements #343 - adds a new --add-link flag (and ADD_LINK env var) that lets users display labeled links (status page, support contact, privacy policy, etc.) on all error pages.

Alongside the new feature, the Helm chart's config.addCode field was refactored from a raw string to an array of objects for consistency with the new config.addLink field. This is a breaking change for Helm chart users who already use config.addCode.

Additionally, I added the homepage link to all built-in templates.

The cats template has been rewritten

⚠️ Breaking change - Helm chart: config.addCode format

This affects you only if you use the Helm chart AND have config.addCode set in your values.

Before (raw string)

config:
  addCode: |
    499=Client Closed Request|The client closed the connection before the server finished responding.
    4**=Client Error|Something went wrong on the client side.

After (array of objects)

config:
  addCode:
    - code: "499"
      message: "Client Closed Request"
      description: "The client closed the connection before the server finished responding"
    - code: "4**"
      message: "Client Error"
      description: "Something went wrong on the client side"

Each item must have code (required) and message (required); description is optional. All values are strings. The chart serializes the array to the CODE=MESSAGE|DESCRIPTION wire format that the application expects - you do not need to know the internal format.

New Helm field: config.addLink

config:
  addLink:
    - {label: "Status Page", url: "https://status.example.com"}
    - {label: "Contact Support", url: "https://example.com/contact"}
    - {label: "Privacy Policy", url: "https://example.com/privacy"}