Case-insensitive username check

[mdawsonuk]

Resolves #2581 by ensuring that auth checks for the username are performed by doing a lower-case comparison.

A test case was also added in auth_test.go to ensure that the case-insensitive check is correctly performed.

[codecov-commenter]

Codecov Report

Merging #4431 (112d05f) into master (3603b1f) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #4431   +/-   ##
=======================================
  Coverage   42.53%   42.53%           
=======================================
  Files         100      100           
  Lines       11693    11693           
=======================================
  Hits         4974     4974           
  Misses       6188     6188           
  Partials      531      531           

Impacted Files	Coverage Δ
internal/home/auth.go	52.87% <100.00%> (ø)

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[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[mdawsonuk]

Not sure why this is being marked as wontfix

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[ainar-g]

(For context, see https://github.com/AdguardTeam/AdGuardHome/issues/2581#issuecomment-1359547557.)