Fix duplicate connection/logs in Webdev

[jyameo]

Fix duplicate app logs on page refresh in Webdev
Fixes: #2604

Problem
Each page refresh created duplicate stdout listeners for the same app, causing exponential log duplication (1 log → 2 logs → 3 logs, etc.).

Root Cause
app_domain.dart created new VM service stdout listeners on every dwds.connectedApps event without checking if listeners already existed for the same appId.

Solution
Added simple tracking to prevent duplicate listeners:

• Track active appIds in a Set _activeListeners
• Only create stdout listeners if not already listening for that appId
• Clean up tracking when connections end

Changes
app_domain.dart: Added listener deduplication with minimal code changes

Result

• Single log event per refresh (no more duplicates)
• All debugging functionality preserved
• No breaking changes

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