Show a spinner for outstanding requests in the browser preview pane

[brandonpayton]

Summary:

• Track outstanding service-worker HTTP bridge requests from the kernel worker through session state.
• Show the restored URL-bar spinner while preview requests are pending.
• Move routine [bridge] request notices to console.debug so they appear as verbose logs; failures still warn.

Verification:

• npm run build (apps/browser-demos)
• ./host/node_modules/.bin/vitest run web-libs/kandelo-session/test/kandelo-session.test.ts --config /dev/null
• git diff --check
• curl -I http://127.0.0.1:5410/pages/kandelo/?demo=wordpress-sqlite

[github-actions]

prepare-merge: runtime/materialization tests passed against the synthetic PR merge; package staging and durable package publishing were skipped. merge-gate=success posted on PR HEAD and squash auto-merge enabled.