feat: add per-section host field for multi-host support

[phgermanov]

Summary

Add per-section host field to support querying multiple GitHub hosts (github.com and GitHub Enterprise) in the same dashboard.

Closes #737

Example config

prSections:
  - title: Enterprise PRs
    filters: is:open author:@me
    host: github.enterprise.com
  - title: GitHub.com PRs  
    filters: is:open author:@me
    # no host = default

How did you test this change?

• Added unit tests for host field parsing in parser_test.go
• All existing tests pass (go test ./...)
• Manual testing with multi-host config

Images/Videos

N/A - no UI changes, configuration-only feature

[dlvhdr]

This is great! Can you also add it under the docs at docs/src/content/docs/configuration/pr-section.mdx and docs/src/content/docs/configuration/issue-section.mdx and docs/src/content/docs/configuration/notification-section.mdx?

[phgermanov]

@dlvhdr thanks, docs are added.