feat: add set text plain and use premailer to set css inline

[akfaiz]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 76.84211% with 22 lines in your changes missing coverage. Please review.

Project coverage is 89.66%. Comparing base (1f1d517) to head (cce098e).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
message.go	83.09%	8 Missing and 4 partials ⚠️
mailer.go	68.42%	4 Missing and 2 partials ⚠️
config.go	20.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #3      +/-   ##
==========================================
- Coverage   90.94%   89.66%   -1.28%     
==========================================
  Files           4        4              
  Lines         265      329      +64     
==========================================
+ Hits          241      295      +54     
- Misses         19       25       +6     
- Partials        5        9       +4     

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