auth.login_path: let a site serve its own login page #159

Problem

The content-gating docs described a loginPath option (custom login page route). It never existed: /auth/login is hard-coded in the gating redirect (src/auth/gating.ts), the mounted routes (src/auth/mount.ts), the password-login error redirect (src/auth/password-login.ts) and the login form. The docs are corrected in duneorg/dune-docs#30. A site with its own login or join page (e.g. a members site with a branded entry page) has no way to send visitors there.

Proposal

site.yaml:

auth:
  login_path: /join   # default: /auth/login

• Gating redirects (page gate, mediaDenialResponse, anything else that builds /auth/login?next=) go to login_path with ?next=.
• GET /auth/login redirects to login_path when it is set, preserving the query (next, error). Decided: it redirects rather than still rendering, so there is one login page.
• Everything else under /auth/* stays mounted: POST /auth/login, OAuth callbacks, magic-link verification, logout. The custom page posts to them.
• Failures that redirect back to the login page (?error=invalid etc.) go to login_path.
• The site provides the page at login_path itself (a normal Dune page or TSX route).

Edge cases to settle in the PR

• Validate the value: a local absolute path (starts with a single /), not an external URL, not //host.
• login_path equal to /auth/login (or unset) is the current behaviour, with no redirect loop.
• login_path must not itself be gated: it is exempt from default_roles (Page gating: inherit roles: from ancestors, add a site-wide default, and apply it to co-located media #157/Inherited and site-default page gating, gated media (#157) #158), or the config check warns, otherwise visitors loop.
• Docs: restore a short "Login redirect" section on the content-gating page and a config-reference row once it ships.

Related: #157, #158.