Fix repo URL width calculation.

[dominicdoty]

Previous code only used description width to calculate remaining space.

At that point the description has already been merged into the header so the header can be used to calculate remaining width correctly.

Fixes #521

E.G.

Before:

After:

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 51.70%. Comparing base (b06b555) to head (e0e840f).
Report is 111 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/ui/pages/repo/repo.go	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #637      +/-   ##
==========================================
- Coverage   51.96%   51.70%   -0.26%     
==========================================
  Files         157      159       +2     
  Lines       13454    13552      +98     
==========================================
+ Hits         6991     7007      +16     
- Misses       5891     5973      +82     
  Partials      572      572              

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[aymanbagabas]

Thank you @dominicdoty !