feat: AI chat UI improvements

[richardshiue]

please don't merge before launch review is passed.

launch review fixes (all visual, no tests required):

• too little padding between messages from the same user (changed to 8px between message)
• too little padding when the window is constrained on desktop (changed to 32px on either side)
• no margins for related questions on mobile (now use 16px on either side)
• no text overflow for related questions
• text misalignment in sources found segment (use 12px font size as per figma)
• user message bubble too narrow (now use 1:5 flex)
• additional padding at beginning of mobile input text field (6px)

welcome.mp4

Feature Preview

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PR Checklist

• My code adheres to AppFlowy's Conventions
• I've listed at least one issue that this PR fixes in the description above.
• I've added a test(s) to validate changes in this PR, or this PR only contains semantic changes.
• All existing tests are passing.

[github-actions]

🥷 Ninja i18n – 🛎️ Translations need to be updated

Project /project.inlang

lint rule	new reports	level	link
Missing translation	219	warning	contribute (via Fink 🐦)
Message without source	3	warning	contribute (via Fink 🐦)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 65.11%. Comparing base (7ecf213) to head (2a5605e).
Report is 250 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6760      +/-   ##
==========================================
- Coverage   74.41%   65.11%   -9.30%     
==========================================
  Files         245      273      +28     
  Lines        6566    10572    +4006     
  Branches     1057     1413     +356     
==========================================
+ Hits         4886     6884    +1998     
- Misses       1628     3517    +1889     
- Partials       52      171     +119     

Flag	Coverage Δ
appflowy_web_app	65.11% <ø> (-9.30%)	⬇️

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