Say what each action did in a one-line notice (closes #383)

Every action on the webhook pages, and signing out, redirects with a
fixed notice code in the URL, and the page it lands on shows one line
saying what was done. noticeFor maps codes to fixed text and an
unknown code shows nothing, so nothing from the URL is echoed. One
partial, templates/notice.html, shows the line under the navbar on
every page; the error page shows none.

Replay and resubmit use the same codes and partial in place of their
own query parameters and event log banners.

Model: opus-5-5

internal/handlers/handlers.go

@@ -97,10 +97,10 @@ type Handlers struct {
 
 // parsePageTemplate parses a page-specific template set from the
 // embedded FS. Each page template is combined with the shared
-// base, htmlheader, and navbar templates, and with any further files
-// the page includes. The page file must be listed first so that its
-// root action ({{template "base" .}}) becomes the template set's entry
-// point.
+// base, htmlheader, navbar and notice templates, and with any further
+// files the page includes. The page file must be listed first so that
+// its root action ({{template "base" .}}) becomes the template set's
+// entry point.
 func parsePageTemplate(
 	pageFile string, included ...string,
 ) *template.Template {
@@ -109,6 +109,7 @@ func parsePageTemplate(
 		"base.html",
 		"htmlheader.html",
 		"navbar.html",
+		"notice.html",
 	}, included...)
 
 	return template.Must(
@@ -209,11 +210,13 @@ func (s *Handlers) renderError(
 	// served outside the routes where NoCache runs.
 	w.Header().Set("Cache-Control", "no-store")
 
+	// No notice: one would say an action worked above a page saying
+	// the request failed.
 	data := s.pageData(r, map[string]any{
 		"Status":     status,
 		"StatusText": http.StatusText(status),
 		"Message":    errorPageText(status),
-	})
+	}, nil)
 
 	var buf bytes.Buffer
 
@@ -267,6 +270,7 @@ type templateDataWrapper struct {
 	User      *UserInfo
 	CSRFToken string
 	Version   string
+	Notice    *notice
 	Data      any
 }
 
@@ -311,12 +315,15 @@ func (s *Handlers) renderTemplate(
 		return
 	}
 
-	s.executeTemplate(w, r, tmpl, s.pageData(r, data))
+	s.executeTemplate(w, r, tmpl, s.pageData(r, data, noticeFor(r)))
 }
 
 // pageData adds the fields the shared layout renders to a page's own
-// data.
-func (s *Handlers) pageData(r *http.Request, data any) any {
+// data. The layout shows the notice, when there is one, above the
+// page.
+func (s *Handlers) pageData(
+	r *http.Request, data any, pageNotice *notice,
+) any {
 	userInfo := s.getUserInfo(r)
 	csrfToken := middleware.CSRFToken(r)
 
@@ -330,6 +337,7 @@ func (s *Handlers) pageData(r *http.Request, data any) any {
 		m["User"] = userInfo
 		m["CSRFToken"] = csrfToken
 		m["Version"] = version
+		m["Notice"] = pageNotice
 
 		return m
 	}
@@ -338,6 +346,7 @@ func (s *Handlers) pageData(r *http.Request, data any) any {
 		User:      userInfo,
 		CSRFToken: csrfToken,
 		Version:   version,
+		Notice:    pageNotice,
 		Data:      data,
 	}
 }