fix: use dc:creator instead of author for item authors

[Zerthick]

Description

Use <dc:creator> instead of <author> for item authors in the RSS 2.0 feed.

The RSS 2.0 spec requires <author> to be an email address. Human-readable names should use <dc:creator> from the Dublin Core namespace (which is already declared in the feed).

Changes

• Change <author> to <dc:creator> in rss.xml.jinja2 template
• Update XSL HTML preview to read dc:creator (items) and managingEditor (channel)
• Add test verifying <dc:creator> usage and absence of <author> in items

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Split from: #451

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💡 AI Contribution Level: 8 — Based on the VisiData AI contribution scale, this was an AI-assisted contribution where the AI performed the code splitting and PR creation with minimal human guidance.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 83.16%. Comparing base (1b5e630) to head (7c34564).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #455   +/-   ##
=======================================
  Coverage   83.16%   83.16%           
=======================================
  Files          11       11           
  Lines         784      784           
  Branches      133      133           
=======================================
  Hits          652      652           
  Misses         86       86           
  Partials       46       46           

Flag	Coverage Δ
unittests	83.16% <ø> (ø)

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