fix: resolve 'Processando audio...' stuck forever on audio messages

[ti-codetec]

Summary

Fixes the bug where "Processando áudio..." placeholder stays spinning forever when sending audio messages, requiring a page reload to clear it.

Root Causes

Three independent issues combined to cause this:

1. session-viewer.html missing transcription handler (primary cause)

The session viewer page (sub-agent sessions) had no WebSocket handler for transcription events. When audio was transcribed in web.ts, the result was sent as a role: "system" message via broadcastToSessionSubscribers, but appendMessage() silently discards system messages (if (role === 'system') return). The placeholder was never updated.

2. DOM audio URL not updated when echo is suppressed

When the user sends audio, the <audio> element is rendered with a local blob: URL. The server echo brings the persistent /audio/xxx URL, but the echo is suppressed by pendingUserEcho. The sessionMessages store was updated but the DOM was not — so when the transcription event arrived with /audio/xxx, URL matching failed. The fallback (find last .processing element) worked in some cases but was fragile.

3. Error/abort paths didn't clear the placeholder

When audio transcription failed (Gemini unavailable, transcription error, or request aborted), agent.ts returned an error message but never sent a transcription event, leaving the placeholder spinning forever.

Changes

File	Change
session-viewer.html	Added handleTranscription() function and 'transcription' case in WebSocket handler
web.ts	Added sendSessionTranscription() method; replaced system message broadcast with proper transcription event for session viewers; also send transcription on error path
main-session-viewer.html	Update <audio> src from blob: to server URL when echo is suppressed
web-ui.html	Same DOM URL update when echo is suppressed
agent.ts	Send transcription event on all error/abort/unavailable paths so placeholder is always cleared

Testing

• TypeScript compiles with zero errors
• JavaScript syntax validated for all 3 HTML files
• Covers all scenarios: successful transcription, Gemini unavailable, transcription failure, request abort, and local blob URL mismatch