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/middleware/middleware.go

@@ -6,6 +6,7 @@ import (
 	"log/slog"
 	"net"
 	"net/http"
+	"net/url"
 	"sync"
 	"time"
 
@@ -366,6 +367,30 @@ func (s *Middleware) CORS() func(http.Handler) http.Handler {
 	}
 }
 
+// NextParam is the query parameter on the login redirect, and the
+// login form field, that holds the page to return to after login.
+const NextParam = "next"
+
+// MaxNextBytes bounds the NextParam value. The login page writes it
+// into its form, and every page is rendered into a buffer first, so
+// without a bound a request would choose the size of that buffer.
+const MaxNextBytes = 2048
+
+// loginURL is the login page RequireAuth redirects to. A GET carries
+// its own path and query in NextParam so that logging in returns to
+// it, unless they are longer than MaxNextBytes; loginDestination in
+// the handlers package checks whether that value is safe to follow.
+// Other methods carry nothing, since a redirect cannot repeat them.
+func loginURL(r *http.Request) string {
+	next := r.URL.RequestURI()
+
+	if r.Method != http.MethodGet || len(next) > MaxNextBytes {
+		return "/pages/login"
+	}
+
+	return "/pages/login?" + url.Values{NextParam: {next}}.Encode()
+}
+
 // RequireAuth returns middleware that checks for a valid session.
 // Unauthenticated users are redirected to the login page.
 func (s *Middleware) RequireAuth() func(http.Handler) http.Handler {
@@ -381,7 +406,7 @@ func (s *Middleware) RequireAuth() func(http.Handler) http.Handler {
 					"error", err,
 				)
 				http.Redirect(
-					w, r, "/pages/login", http.StatusSeeOther,
+					w, r, loginURL(r), http.StatusSeeOther,
 				)
 
 				return
@@ -409,7 +434,7 @@ func (s *Middleware) RequireAuth() func(http.Handler) http.Handler {
 					),
 				)
 				http.Redirect(
-					w, r, "/pages/login", http.StatusSeeOther,
+					w, r, loginURL(r), http.StatusSeeOther,
 				)
 
 				return