feat(ui, samples): add a badge count to the call buttons

[renefloor]

Summary

• CallControlButton, CallFeatureButton and StreamCallButtonBadge take one badge, a sealed CallControlBadge, in place of showErrorBadge. CallControlErrorBadge draws the error badge; CallControlNotificationBadge draws a count as a StreamBadgeNotification, optionally in a StreamBadgeNotificationType. A zero count draws nothing.
• StreamParticipantsButton (and so the lobby) uses it with the neutral badge instead of badging itself in accentSuccess.
• The sample app's BadgedCallOption is removed; its participants and chat buttons pass a CallControlNotificationBadge, and the participants count is neutral.

Testing

• New StreamCallButtonBadge tests for the count and its type.
• The participants and lobby button tests tap the StreamButton, since the badge's box fills the test surface when the button is pumped on its own.
• No golden in the lobby shows a count.

🤖 Generated with Claude Code

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[codecov]

Codecov Report

❌ Patch coverage is 64.28571% with 10 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (v2@002747a). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...ib/src/call_controls/stream_call_button_badge.dart	50.00%	10 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##             v2    #1376   +/-   ##
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  Coverage      ?   37.93%           
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  Files         ?      395           
  Lines         ?    30171           
  Branches      ?        0           
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  Hits          ?    11445           
  Misses        ?    18726           
  Partials      ?        0           

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