## Why

Profile settings correctly render an uploaded custom background, but the page header metric and background preset controls still label the built-in preset as the current background. This makes users think the uploaded background was ignored when the built-in preset is actually only the fallback after upload removal.

## What Changes

- Show an uploaded custom background as the active background in profile settings summary metrics.
- Treat the selected built-in background preset as a fallback while a custom background is active.
- Label the fallback preset explicitly and avoid marking it as the pressed/active visible background.
- Keep uploaded background precedence and existing theme visual direction unchanged.

## Impact

- Affected root: `platform_web/`.
- Expected files: `platform_web/pages/ProfileSettingsPage.tsx`, `platform_web/theme/base.css`, and focused page rendering tests.
- No platform API, persistence schema, authentication, plugin, run, or server management behavior changes.
- Verification: focused frontend tests, `scripts/check-structure.sh`, and `openspec validate fix-custom-background-status --strict`.