fix(watch): close MultiBackend's sync and sources

[skirsten]

MultiBackend.close() only closed its own #runElement signal, leaving the Sync and the two Source instances (constructed as fields, not inside an Effect) alive. Each owns its own Effect with several .run() handlers that never get cleaned up, surfacing as "Signals was garbage collected without being closed" warnings whenever a watch component unmounts.

Close them explicitly, ordered so inner Decoder/Renderer/Emitter (spawned via the #runElement effect) finish before the sources/sync they reference are torn down.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: Organization UI

Review profile: CHILL

Plan: Pro

Run ID: 10f57718-6334-4867-82d1-bfe23fc9dd4c

📥 Commits

Reviewing files that changed from the base of the PR and between 6adf3c3 and 0271f05.

📒 Files selected for processing (1)

• js/watch/src/backend.ts

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Walkthrough

This PR expands the MultiBackend.close() method to actively release internal media and streaming resources. Previously, the method only terminated the signals effect. The updated implementation now additionally closes the private #videoSource and #audioSource fields and terminates the sync component, ensuring complete resource cleanup when the backend is shut down.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: closing MultiBackend's sync and sources, which directly matches the primary fix in the changeset.
Description check	✅ Passed	The description is comprehensive and directly related to the changeset, explaining the problem, the solution, and the cleanup order.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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