feat(ui): Improve slow mode UI

[renefloor]

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Linear: FLU-496

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• I have signed the Stream CLA (required).
• The code changes follow best practices
• Code changes are tested (add some information if not applicable)

Description of the pull request

Currently slow mode didn't disable the composer, you could still type. This PR disables the composer, the attachment button and shows the timer.

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Before	After

Summary by CodeRabbit

• New Features

  • Slow mode now displays a live countdown timer showing remaining seconds, replacing the static label
  • Message input is disabled and the send button is replaced with a countdown display while slow mode is active
  • Updated slow mode labels across all supported languages

• Documentation

  • Updated migration guides and changelog documenting slow mode behavior changes

• Tests

  • Updated message input tests to validate slow mode countdown display and disabled input state

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[codecov]

Codecov Report

❌ Patch coverage is 97.33333% with 2 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (v10.0.0@b73bb71). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...age_composer/message_composer_component_props.dart	75.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             v10.0.0    #2666   +/-   ##
==========================================
  Coverage           ?   66.69%           
==========================================
  Files              ?      404           
  Lines              ?    24137           
  Branches           ?        0           
==========================================
  Hits               ?    16097           
  Misses             ?     8040           
  Partials           ?        0           

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[xsahil03x]

Suggested some improvements, but LGTM