[web] Add Symphony favicon

[sharmila-oai]

Context

Symphony Observability currently uses the browser's generic fallback icon, which makes its control-plane tabs hard to identify.

TL;DR

Add a custom, cache-busted favicon to the Symphony Observability dashboard.

Summary

• Add a generated 128x128 transparent PNG mark for Symphony.
• Serve the favicon through the existing embedded static-asset controller.
• Add a content digest to the favicon URL to avoid stale browser caches.
• Verify the favicon markup, route response, content type, and binary body.

Alternatives

• Keep the generic browser icon. This leaves Symphony tabs difficult to distinguish.
• Embed a data URI in the layout. A normal asset route is easier to cache, test, and replace.

Test Plan

• make -C elixir all
• mix test test/symphony_elixir/extensions_test.exs
• Browser preview at http://127.0.0.1:4022/

[sharmila-oai]

@codex review

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