fix: align textarea placeholder and text with the surroundings

[MartinCupela]

🎯 Goal

Closes REACT-919

Fixes:

• An unexpected grey line appears inside the thread composer input field, likely a stray border or divider element that shouldn't be visible.
• The placeholder text is not vertically centered within the input field and needs to be repositioned.
• The command pill has too much horizontal padding; left and right should be reduced.

[github-actions]

Size Change: +4 B (0%)

Total Size: 441 kB

Filename	Size	Change
./dist/cjs/index.js	236 kB	-13 B (-0.01%)
./dist/css/index.css	46.9 kB	+17 B (+0.04%)

ℹ️ View Unchanged

Filename	Size
./dist/audioProcessing-BbOs2wMd.js	1.32 kB
./dist/cjs/emojis.js	2.96 kB
./dist/cjs/mp3-encoder.js	1.27 kB
./dist/css/v2/emoji-mart.css	1.84 kB
./dist/css/v2/emoji-replacement.css	300 B
./dist/css/v2/index.css	39.4 kB
./dist/css/v2/index.layout.css	22.8 kB
./dist/WithAudioPlayback-myzUS2m6.js	89.1 kB

_{compressed-size-action}

[codecov]

Codecov Report

❌ Patch coverage is 66.66667% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 78.77%. Comparing base (880d12d) to head (a3f1bbc).

Files with missing lines	Patch %	Lines
...c/components/MessageComposer/MessageComposerUI.tsx	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3052      +/-   ##
==========================================
+ Coverage   78.75%   78.77%   +0.02%     
==========================================
  Files         426      426              
  Lines       12110    12110              
  Branches     3927     3929       +2     
==========================================
+ Hits         9537     9540       +3     
+ Misses       2552     2549       -3     
  Partials       21       21              

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