Logging in returns to the page that was asked for (closes #384)

RequireAuth now sends a logged-out GET to /pages/login with its path
and query in a `next` parameter when they fit in 2048 bytes, the same
limit the login page applies. The login form carries it as a hidden
field, and a successful login redirects there when it is a path on
this site; anything else, plain or percent-encoded, goes to `/`, which
leads to the webhook list. A browser already logged in that opens the
login page goes to the same place. The navigation bar on the login
page no longer links to the login page.

Model: opus-5-5

internal/handlers/profile_test.go

@@ -160,7 +160,10 @@ func TestUserRoute_Unauthenticated_RedirectedByMiddleware(t *testing.T) {
 		"handler must not be reached for unauthenticated request",
 	)
 	assert.Equal(t, http.StatusSeeOther, w.Code)
-	assert.Equal(t, "/pages/login", w.Header().Get("Location"))
+	assert.Equal(
+		t, "/pages/login?next=%2Fuser%2Ftestuser",
+		w.Header().Get("Location"),
+	)
 }
 
 // passwordChangeRequest builds a POST request to the password-change