Gzip the text responses

Nothing was compressed: the list page shipped ~123 KB of HTML, CSS and JS in
the clear on every cold load, which is the wrong bill for a phone on mobile
data. A stdlib middleware compresses the four content types this server
actually serves in bulk and leaves woff2 alone, since it is already compressed.

Measured on the live server: style.css 21.8 KB -> 6.5, htmx 50.9 -> 16.4,
filter.js 7.6 -> 2.9.

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

backend/main_test.go

@@ -1,8 +1,10 @@
 package main
 
 import (
+	"compress/gzip"
 	"encoding/json"
 	"fmt"
+	"io"
 	"net/http"
 	"net/http/httptest"
 	"strings"
@@ -196,3 +198,46 @@ func TestPutOmittedStatusPreservesArchivedAndAppliesProgress(t *testing.T) {
 		t.Fatalf("stored last_chapter_num = %v, want 12", got.LastChapterNum)
 	}
 }
+
+func TestGzipCompressesTextNotFonts(t *testing.T) {
+	srv, _ := newWebTestServer(t, webConfig())
+
+	cases := []struct {
+		path string
+		want bool
+	}{
+		{"/static/style.css", true},
+		{"/static/filter.js", true},
+		{"/static/htmx.min.js", true},
+		{"/static/fonts/dm-sans-var-latin.woff2", false},
+	}
+	for _, tc := range cases {
+		req := httptest.NewRequest(http.MethodGet, tc.path, nil)
+		req.Header.Set("Accept-Encoding", "gzip")
+		rr := httptest.NewRecorder()
+		srv.ServeHTTP(rr, req)
+		if rr.Code != http.StatusOK {
+			t.Fatalf("GET %s = %d, want 200", tc.path, rr.Code)
+		}
+		got := rr.Header().Get("Content-Encoding") == "gzip"
+		if got != tc.want {
+			t.Errorf("GET %s Content-Encoding gzip = %v, want %v", tc.path, got, tc.want)
+		}
+		if got {
+			zr, err := gzip.NewReader(rr.Body)
+			if err != nil {
+				t.Fatalf("GET %s: body is not gzip: %v", tc.path, err)
+			}
+			if _, err := io.ReadAll(zr); err != nil {
+				t.Fatalf("GET %s: gzip body did not decode: %v", tc.path, err)
+			}
+		}
+	}
+
+	// A client that does not ask still gets plain bytes.
+	rr := httptest.NewRecorder()
+	srv.ServeHTTP(rr, httptest.NewRequest(http.MethodGet, "/static/style.css", nil))
+	if enc := rr.Header().Get("Content-Encoding"); enc != "" {
+		t.Errorf("Content-Encoding without Accept-Encoding = %q, want empty", enc)
+	}
+}