auth: split dev-mode auth constructor and wire up dev-login/logout UI

Squeamish about New()'s empty-domain-string sentinel for "dev mode, skip
OIDC discovery" - split into New (always makes a real OIDC discovery
call, all params required) and NewDev (no ctx/domain/credentials at all,
since none are used). main.go now branches on cfg.DevAuthEnabled to pick
the right constructor instead of main.go/config.go coordinating on when
it's safe to pass empty strings.

Also finishes out the dev-auth flow this enables: config.Load reads a
DEV_AUTH_ENABLED-aware env file and only requires Auth0 vars when dev
auth is off; a PORT config var replaces the hardcoded :8082; and the nav
UI (layout/index templates, ui router) points login/logout links at
/api/auth/dev-login and a new /api/auth/dev-logout route when dev auth
is enabled, so the whole login/logout loop works locally without a real
Auth0 app.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

domains/authentication/auth.go

@@ -45,7 +45,10 @@ type (
 	}
 )
 
-// New instantiates the *Authenticator.
+// New instantiates an *Authenticator backed by a real Auth0 tenant: it makes
+// an OIDC discovery call against domain, so login/callback/logout are fully
+// functional. Use NewDev instead when DEV_AUTH_ENABLED is set and no real
+// Auth0 app is configured.
 func New(
 	ctx context.Context,
 	db *pgxpool.Pool,
@@ -75,6 +78,18 @@ func New(
 	}, nil
 }
 
+// NewDev instantiates an *Authenticator with no real Auth0 tenant behind it:
+// no OIDC discovery call is made, and Provider/Config are left zero-valued.
+// Only DevLogin is safe to call on the result - Exchange, VerifyIDToken, and
+// GetLogoutURL all assume a real Auth0 setup and will misbehave. Only use
+// this from a route gated on an explicit dev-mode flag.
+func NewDev(db *pgxpool.Pool, logger *logging.Logger) *Authenticator {
+	return &Authenticator{
+		log: logger,
+		db:  db,
+	}
+}
+
 func (a *Authenticator) RunBackgroundCleanup(ctx context.Context) error {
 	for {
 		if _, err := a.db.Exec(ctx, "DELETE FROM oauth_tokens WHERE expiry < NOW()"); err != nil {