feat(ui): brand mark logo in the web UI, panel, and favicon

The mark is the same drawing in three places with three different
colour sources: /static/logo.svg carries a fixed palette because a
favicon has no page to inherit from, the {{define "mark"}} block takes
--ink/--ember so it flips with the theme, and the userscript copy lives
inside the shadow root next to the tokens it uses.

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

backend/web_test.go

@@ -189,7 +189,7 @@ func TestBookmarksAPIStillBearerOnly(t *testing.T) {
 
 func TestStaticAssetsServed(t *testing.T) {
 	srv, _ := newWebTestServer(t, webConfig())
-	for _, path := range []string{"/static/style.css", "/static/htmx.min.js", "/static/filter.js"} {
+	for _, path := range []string{"/static/style.css", "/static/htmx.min.js", "/static/filter.js", "/static/logo.svg"} {
 		rr := httptest.NewRecorder()
 		srv.ServeHTTP(rr, httptest.NewRequest(http.MethodGet, path, nil))
 		if rr.Code != http.StatusOK {