fix: remove duplicate heading on login page

[elias-ba]

Description

This PR fixes a duplicate "Log in" heading on the login page. The LayoutComponents.header component rendered two independent <h1> elements — one for the no-breadcrumbs case and another for the no-current_user case. On the login page both conditions were true, causing the title to appear twice. The fix merges them into a single <h1> with conditional styling.

Closes #4459

Validation steps

1. Visit /users/log_in while logged out; verify only one "Log in" heading appears (large, bold)
2. Visit any authenticated page with breadcrumbs; verify breadcrumbs render normally
3. Visit an authenticated page without breadcrumbs; verify the title renders once in the smaller style

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.32%. Comparing base (14c67e9) to head (c5d6b92).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4460      +/-   ##
==========================================
- Coverage   89.35%   89.32%   -0.04%     
==========================================
  Files         425      425              
  Lines       20222    20221       -1     
==========================================
- Hits        18069    18062       -7     
- Misses       2153     2159       +6     

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