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/routes.go

@@ -1,10 +1,14 @@
 package server
 
 import (
+	"bytes"
+	"crypto/sha256"
+	"encoding/hex"
 	"io/fs"
 	"net/http"
 	"strconv"
 	"strings"
+	"time"
 
 	"jarl/internal/api"
 	"jarl/internal/auth"
@@ -120,6 +124,7 @@ func (s *Server) routes() {
 	// Web UI
 	if s.web != nil {
 		files := http.FileServer(http.FS(s.web))
+		etags := contentETags(s.web)
 		m.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
 			if r.Method != http.MethodGet && r.Method != http.MethodHead {
 				w.Header().Set("Allow", "GET, HEAD")
@@ -134,11 +139,17 @@ func (s *Server) routes() {
 				}
 				w.Header().Set("Content-Type", "text/html; charset=utf-8")
 				w.Header().Set("Cache-Control", "no-cache")
-				w.Write(b)
+				w.Header().Set("ETag", etags["index.html"])
+				http.ServeContent(w, r, "index.html", time.Time{}, bytes.NewReader(b))
 				return
 			}
 			if strings.HasPrefix(r.URL.Path, "/assets/") {
+				// no-cache: the browser asks again each time, but with
+				// the ETag an unchanged file costs a 304 and no body.
 				w.Header().Set("Cache-Control", "no-cache")
+				if tag, ok := etags[strings.TrimPrefix(r.URL.Path, "/")]; ok {
+					w.Header().Set("ETag", tag)
+				}
 				files.ServeHTTP(w, r)
 				return
 			}
@@ -233,3 +244,22 @@ func require(p *auth.Principal, a auth.Action) error {
 	}
 	return nil
 }
+
+// contentETags returns an ETag for every file of the embedded web UI,
+// from its content (embedded files have no modification time).
+func contentETags(fsys fs.FS) map[string]string {
+	tags := map[string]string{}
+	fs.WalkDir(fsys, ".", func(path string, d fs.DirEntry, err error) error {
+		if err != nil || d.IsDir() {
+			return nil
+		}
+		b, err := fs.ReadFile(fsys, path)
+		if err != nil {
+			return nil
+		}
+		sum := sha256.Sum256(b)
+		tags[path] = `"` + hex.EncodeToString(sum[:12]) + `"`
+		return nil
+	})
+	return tags
+}