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.

docs/environment-variables.md

@@ -314,6 +314,7 @@ Audiobook formats to include in search results. ZIP/RAR archives are extracted a
 |----------|-------------|------|---------|
 | `SEARCH_MODE` | How you want to search for and download books. | string (choice) | `universal` |
 | `BOOK_LANGUAGE` | Default language filter for searches. Users can override this for their own account. | string (comma-separated) | `en` |
+| `DEFAULT_CONTENT_TYPE` | Which tab the search page opens on. Users can override this for their own account, and a browser that has already picked a tab keeps its choice. | string (choice) | `ebook` |
 | `AA_DEFAULT_SORT` | Default sort order for search results. | string (choice) | `relevance` |
 | `SHOW_RELEASE_SOURCE_LINKS` | Show clickable release-source links in release and details modals. Metadata provider links stay enabled. | boolean | `true` |
 | `SHOW_COMBINED_SELECTOR` | Show the option to search for and download both a book and audiobook together. | boolean | `true` |
@@ -346,6 +347,16 @@ Default language filter for searches. Users can override this for their own acco
 - **Type:** string (comma-separated)
 - **Default:** `en`
 
+#### `DEFAULT_CONTENT_TYPE`
+
+**Default Content Type**
+
+Which tab the search page opens on. Users can override this for their own account, and a browser that has already picked a tab keeps its choice.
+
+- **Type:** string (choice)
+- **Default:** `ebook`
+- **Options:** `ebook` (Ebook), `audiobook` (Audiobook)
+
 #### `AA_DEFAULT_SORT`
 
 **Default Sort Order**