[Doc] Linux Service improvement

[benoit-cty]

Description

• docs/how-to/linux-service.md:48-73 — Step 3 rewritten to run login/config as your own user, explains why the service user can't, and adds the headless case
• Step 6 now says to copy the values from ~/.codecarbon.config
• . codecarbon/cli/auth.py:172 — always prints the authorization URL, so a failed webbrowser.open (which returns True even when xdg-open bails, as it did for you) is no longer a dead end.
• codecarbon/cli/auth.py:88-98 — _save_credentials turns the raw Errno 13 into a message naming the resolved path and telling you to run from a writable directory.

Motivation and Context

When installing on a new machine following the documentation, it fail.

How Has This Been Tested?

Install on a fresh Ubuntu 26.04.

Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

AI Usage Disclosure

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• 🟥 AI-vibecoded: You cannot explain the logic. Car analogy : the car drive by itself, you are outside it and just tell it where to go.
• 🟠 AI-generated: Car analogy : the car drive by itself, you are inside and give instructions.
• ⭐ AI-assisted. Car analogy : you drive the car, AI help you find your way.
• ♻️ No AI used. Car analogy : you drive the car.

Checklist:

Go over all the following points, and put an x in all the boxes that apply.

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the docs/how-to/contributing.md document.
• I have added tests to cover my changes.
• All new and existing tests passed.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.58%. Comparing base (2c1b435) to head (cc4a808).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1401      +/-   ##
==========================================
+ Coverage   91.54%   91.58%   +0.04%     
==========================================
  Files          49       49              
  Lines        5096     5100       +4     
==========================================
+ Hits         4665     4671       +6     
+ Misses        431      429       -2     

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[inimaz]

Thanks @benoit-cty !