ba30345

LXManga: drop MangaParserAuthProvider so the 'Đăng nhập' row goes away

The user pointed out that login and 'solve captcha' are two different
features, and that showing a 'Đăng nhập' (Sign in) row for a site that
has no account system at all is actively misleading. lxmanga.space
only has the Cloudflare Turnstile captcha — there is no username,
no password, no 'Đã đăng nhập' state to ever reach.

The fix: drop the [MangaParserAuthProvider] interface entirely so
the host app's source-settings screen has nothing to render for
this parser. Captcha solving is the only auth flow, and the user
hits it automatically the first time the catalog request returns
the 'Just a moment…' page — [fetchDocument] detects the response,
hands the URL to [MangaLoaderContext.requestBrowserAction], and
the host app surfaces its own captcha prompt (custom tab, dialog,
whatever it normally does for 'solve this captcha').

Compared to the previous design (78ee241):
  * No more 'Đăng nhập' row in source settings — the user is not
    asked to 'sign in' anywhere.
  * No more [authUrl] in the in-app WebView — the only thing the
    app does is run [requestBrowserAction] when the catalog 403s.
  * No more [isAuthorized] / [getUsername] / [MangaParserAuthProvider]
    at all. The OkHttp cookie jar check that used to live in
    [isAuthorized] is gone too: with no auth provider the app
    has no way to invoke us, so there is no need to expose state.
    The captcha prompt is the *only* entry point.

The chapter reader (PAGES_SCRIPT + evaluateJs) is unchanged — the
obfuscated image-URL builder still has to run inside a browser
context.

Tests: 24/24 pass. The harness covers the auth state, the static
request headers, and all of [getPages]'s JSON / error paths.
The live catalog flow is exercised by the CI runner.