Fix JoinAttrs not handling nested Groups

[maragubot]

Summary

• JoinAttrs only flattened one level of g.Group, so attributes inside nested groups were rendered as duplicate attributes instead of being joined
• Replaced the two-branch loop (one for groups, one for non-groups with duplicated logic) with a single recursive processNode closure that flattens groups to any depth

Test plan

• Added test for joining classes across nested groups
• Added test for preserving non-attribute nodes in nested groups
• Added test for not joining attributes inside element children
• Added test proving element-type nodes are never rendered during checking
• All existing tests pass
• Linter passes with zero issues

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (090d0fd) to head (4dbf825).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #293   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
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  Files            6         6           
  Lines          639       635    -4     
=========================================
- Hits           639       635    -4     

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