fix: support app background script leaking text onto page
The support background.html had its own <script> tags, but app.html
already wraps background templates in <script> tags. The nested
</script> caused the browser to close the outer tag early, rendering
the catch block as visible text. Also fixed start() → initialize()
to match AppServices API so polling actually runs.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>