Detect and fill login fields inside open shadow roots

[blacksun80]

Problem

Kee finds fields via document.forms and getElementsByTagName("input"), so it never sees inputs that live inside a shadow root. More and more login pages are built from web components.

A concrete example is the new GMX login at https://auth.gmx.net/login, a two-step form:

1. The first step only shows the username field (<input type="email" autocomplete="username">, no name). Kee ignored this form because it has no password field, unless the user whitelisted the field manually.
2. After clicking "Next" the password field appears. It sits inside the open shadow root of a <lux-password> custom element, which is placed inside the login <form>. Kee never found it.

Similar reports on the forum: Chase, auth.max.com, web.de/gmx.de.

Solution

Commit 1: Find and fill fields in open shadow roots

• New helper src/page/shadowDom.ts finds open shadow roots (including nested ones) and the inputs inside them.
• FormUtils.getFormFields also returns the inputs from shadow roots whose outermost host is inside the form. Username and password are then still detected and filled as one form.
• Shadow root inputs whose host is not inside any form go into the existing pseudo form for orphaned fields.
• The MutationObserver in page.ts also observes open shadow roots (both those present at startup and those added later), so fields rendered into a shadow root later on still trigger a rescan.

Commit 2: Treat autocomplete="username" fields as login fields

A text field with autocomplete="username" is now preferred as the username field and makes a form interesting even without a password field, as a whitelisted field would. This covers the first step of multi-step logins without per-site configuration. It is a separate commit because it is a (small) behaviour change beyond shadow DOM support; it can be dropped on its own if you'd rather not include it.

Limitations

• Closed shadow roots are not accessible to extensions and remain unsupported.
• A shadow root attached to an element that is already in the page doesn't cause a DOM mutation, so it is only picked up on the next regular scan.
• Every scan now walks the page once to find shadow roots (using a native TreeWalker).
• Shadow root inputs are appended after the form's own elements, which matters only for the "field before the password" username fallback.
• Submit buttons inside shadow roots are not searched for.

Testing

• New Vitest tests (jsdom) in src/tests/shadowDom.test.ts and src/tests/formsUtils.test.ts: open, nested and closed shadow roots, hosts inside and outside forms, a <lux-password>-like form, and autocomplete="username". The existing ConfigManager.test.ts failures are unrelated and also happen on master.
• tsc --noEmit and eslint report no new problems.
• Manually tested with the Chrome MV3 build (npm run build-chrome:beta) loaded unpacked in Edge with KeePassRPC on https://auth.gmx.net/login: the username is filled on step 1 without any whitelist entry and the password is filled on step 2.

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