feat(search): add a configurable default content type (#1371)

Closes #1018.

Worth correcting the issue first: the search tab is not hardcoded.
`useContentTypePreferences` has persisted the user's choice to
localStorage since #564, so a browser that has picked a tab already
keeps it. What is missing is the other half the issue asks for, a
default for a browser that has stored nothing, and a per-user override.

`DEFAULT_CONTENT_TYPE` is a user overridable select next to
`BOOK_LANGUAGE`, so it follows the same path: global value in Settings,
per-user value in Search Preferences, resolved with `user_id` in
`/api/config`. The frontend uses it only when this browser has no stored
choice, which is captured before the existing effect writes one, so
nothing changes for anyone who has already picked a tab.

The resolution is a pure function in `utils/contentTypePreference.ts`
rather than logic inside the hook, since vitest here has no jsdom and
the existing tests cover resolvers like `resolveDefaultLanguageCodes`
the same way.

One small move in `App.tsx`: the `config` state was declared below the
hook that now reads it, so it moved above it.

## Verification

- `tests/core/test_config_api.py`: the payload carries
`default_content_type` and reads it with the user's id.
- `tests/core/test_admin_users_api.py`: the key appears in the per-user
search preferences list.
- `src/frontend/src/tests/contentTypePreference.test.ts`: a stored tab
wins, combined mode survives, the server default applies when nothing is
stored, and an unrecognised value falls back to ebook.
- Python suite (3163) and frontend suite (201) green, plus ruff, ruff
format, basedpyright, vulture, tsc, oxlint, oxfmt and the production
build.

tests/core/test_admin_users_api.py

@@ -1367,6 +1367,7 @@ class TestAdminSearchPreferences:
         assert data["keys"] == [
             "SEARCH_MODE",
             "BOOK_LANGUAGE",
+            "DEFAULT_CONTENT_TYPE",
             "SHOW_COMBINED_SELECTOR",
             "FORCE_COMBINED_SEARCH",
             "METADATA_PROVIDER",