fix(ui): disable microphone Opus DTX

[staging-devin-ai-integration]

Summary

• Explicitly disable Opus DTX for the browser microphone publisher. The upgraded publisher suppresses regular frames during silence, while StreamKit’s clocked decoder/mixer path currently interprets those expected gaps as repeated slow-input stalls.
• Restore continuous 20 ms microphone delivery for v0.6.0 without weakening the mixer’s 100 ms warning or hiding state transitions in the Monitor UI.
• Add publish-path regression coverage for the explicit codec policy.

Review & Validation

• Verify a silent browser microphone no longer repeatedly toggles slow_input_timeout.
• Confirm a genuine interruption of continuous audio delivery still triggers the configured mixer timeout.

Notes

This is the immediate mitigation tracked in #681. DTX-aware clocked decoding/silence generation remains a future enhancement, so the issue should be updated rather than automatically closed.

Link to Devin session: https://staging.itsdev.in/sessions/738c0703efe441cb80769d070c318be6
Open in Devin Desktop: https://staging.itsdev.in/desktop/session/738c0703efe441cb80769d070c318be6?variant=devin-insiders
Requested by: @streamer45

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Devin Review

Status	Commit
🟢 Reviewed	5c1f3e8

[staging-devin-ai-integration]

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[staging-devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.

Debug

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 85.32%. Comparing base (717444b) to head (5c1f3e8).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #682      +/-   ##
==========================================
- Coverage   85.33%   85.32%   -0.02%     
==========================================
  Files         249      249              
  Lines       76190    76190              
  Branches     2329     2452     +123     
==========================================
- Hits        65019    65007      -12     
- Misses      11165    11177      +12     
  Partials        6        6              

Flag	Coverage Δ
backend	85.36% <ø> (-0.02%)	⬇️
ui	84.94% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Components	Coverage Δ
core	85.79% <ø> (ø)
engine	83.76% <ø> (ø)
api	91.14% <ø> (ø)
nodes	84.96% <ø> (-0.05%)	⬇️
server	85.26% <ø> (ø)
plugin-native	84.79% <ø> (ø)
plugin-wasm	95.41% <ø> (ø)
ui-services	86.54% <ø> (ø)
ui-components	70.18% <ø> (ø)

Files with missing lines	Coverage Δ
ui/src/stores/streamStoreHelpers.ts	67.61% <ø> (ø)

... and 1 file with indirect coverage changes

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