Fix the logo's pop-in with a traced vector logo

The logo was an 880 KB SVG wrapping a 1254px PNG, downloaded again on
every page load. Keep that PNG as the master (artwork/logo.png) and have
scripts/logo.py trace it with potrace into a 6 KB logo.svg (favicon,
README) and logo.js, which brand.js draws inline in the header and on the
login page so the logo arrives with the page. Remove the unused
logo-white.svg.

Static files now carry content ETags, so reloads get 304s instead of
downloading the UI again.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/static_test.go

@@ -0,0 +1,36 @@
+package server
+
+import (
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+	"testing/fstest"
+)
+
+// Static files carry an ETag, so a browser's revalidation of an unchanged
+// file is a 304 without a body.
+func TestStaticETags(t *testing.T) {
+	web := fstest.MapFS{
+		"index.html":     {Data: []byte("<!doctype html><title>Jarl</title>")},
+		"assets/logo.js": {Data: []byte("export const x = 1;\n")},
+	}
+	s := &Server{cfg: Config{}, log: slog.New(slog.NewTextHandler(io.Discard, nil)), mux: http.NewServeMux(), limiter: newRateLimiter(), web: web}
+	s.routes()
+	for _, path := range []string{"/", "/assets/logo.js"} {
+		w := httptest.NewRecorder()
+		s.mux.ServeHTTP(w, httptest.NewRequest("GET", path, nil))
+		tag := w.Header().Get("ETag")
+		if w.Code != 200 || tag == "" || w.Body.Len() == 0 {
+			t.Fatalf("%s: %d etag %q", path, w.Code, tag)
+		}
+		r := httptest.NewRequest("GET", path, nil)
+		r.Header.Set("If-None-Match", tag)
+		w = httptest.NewRecorder()
+		s.mux.ServeHTTP(w, r)
+		if w.Code != http.StatusNotModified || w.Body.Len() != 0 {
+			t.Errorf("%s revalidated: %d, %d bytes", path, w.Code, w.Body.Len())
+		}
+	}
+}