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

A logged-out GET of an admin page now redirects to /pages/login with its path and query in a next parameter, when they fit in 2048 bytes. The login form carries next as a hidden field; a successful login redirects there, and a failed one shows the page again with the same next. A POST still redirects to plain /pages/login.

The value is client-chosen, so every read of it goes through one check: after percent-decoding it must start with exactly one / and contain no backslash or control character; anything else becomes /. gosec's open-redirect rule is suppressed on the two redirects that follow it. The login page's navigation bar no longer links to itself.

Model: opus-5-5

internal/handlers/handlers.go

@@ -36,6 +36,9 @@ const (
 	tmplKeyError = "Error"
 	// tmplKeyWebhook is the template data key for a webhook.
 	tmplKeyWebhook = "Webhook"
+	// tmplKeyNext is the template data key for the page to return
+	// to after login.
+	tmplKeyNext = "Next"
 )
 
 // errInvalidPassword is returned when a password does not match.