feat: 站点媒体库与移动端底部导航，服务测试补强

- 新增媒体库：media_library/media_thumbs 服务（WebP 缩略图）、管理端 media 页面
- 移动端底部导航 MobileTabBar 替换 MobilePostBar
- 旧数据导入增强与测试、路由与登录会话/板块侧边栏测试补强
- site-doc 组件精简（移除 Breadcrumb），文档新增迁移公告说明

backend/service/upload_webp_test.go

@@ -0,0 +1,172 @@
+package service
+
+import (
+	"bytes"
+	"image"
+	"image/color"
+	"image/gif"
+	"image/jpeg"
+	"image/png"
+	"os"
+	"path/filepath"
+	"strings"
+	"testing"
+
+	"github.com/freefire/jiang13-bbs/model"
+	webpenc "github.com/gen2brain/webp"
+	"github.com/glebarez/sqlite"
+	xwebp "golang.org/x/image/webp"
+	"gorm.io/gorm"
+)
+
+func newUploadTestService(t *testing.T) (*UploadService, string) {
+	t.Helper()
+	db, err := gorm.Open(sqlite.Open(":memory:"), &gorm.Config{})
+	if err != nil {
+		t.Fatalf("open sqlite: %v", err)
+	}
+	if err := db.AutoMigrate(&model.Attachment{}); err != nil {
+		t.Fatalf("migrate: %v", err)
+	}
+	dir := t.TempDir()
+	s := NewUploadService(db, dir)
+	if err := s.EnsureDir(); err != nil {
+		t.Fatalf("ensure dir: %v", err)
+	}
+	return s, dir
+}
+
+func storedUploadPath(t *testing.T, dir, url string) []byte {
+	t.Helper()
+	rel := strings.TrimPrefix(url, "/uploads/")
+	data, err := os.ReadFile(filepath.Join(dir, filepath.FromSlash(rel)))
+	if err != nil {
+		t.Fatalf("read stored file %s: %v", url, err)
+	}
+	return data
+}
+
+func TestSaveImageTranscodesJPEGToWebP(t *testing.T) {
+	s, dir := newUploadTestService(t)
+
+	src := image.NewRGBA(image.Rect(0, 0, 48, 36))
+	for y := 0; y < 36; y++ {
+		for x := 0; x < 48; x++ {
+			src.Set(x, y, color.RGBA{uint8(x * 5), uint8(y * 7), 128, 255})
+		}
+	}
+	var in bytes.Buffer
+	if err := jpeg.Encode(&in, src, nil); err != nil {
+		t.Fatalf("encode jpeg: %v", err)
+	}
+
+	att, err := s.SaveImage(1, bytes.NewReader(in.Bytes()))
+	if err != nil {
+		t.Fatalf("SaveImage: %v", err)
+	}
+	if !strings.HasSuffix(att.URL, ".webp") || att.MIME != "image/webp" {
+		t.Fatalf("want webp attachment, got url=%s mime=%s", att.URL, att.MIME)
+	}
+	if att.Width != 48 || att.Height != 36 {
+		t.Fatalf("dims changed: %dx%d", att.Width, att.Height)
+	}
+
+	img, err := xwebp.Decode(bytes.NewReader(storedUploadPath(t, dir, att.URL)))
+	if err != nil {
+		t.Fatalf("decode stored webp: %v", err)
+	}
+	if got := img.Bounds(); got.Dx() != 48 || got.Dy() != 36 {
+		t.Fatalf("stored webp dims: %v", got)
+	}
+}
+
+func TestSaveImageTranscodesPNGLosslessKeepsAlpha(t *testing.T) {
+	s, dir := newUploadTestService(t)
+
+	src := image.NewNRGBA(image.Rect(0, 0, 24, 24))
+	for y := 0; y < 24; y++ {
+		for x := 0; x < 24; x++ {
+			if y < 12 {
+				src.SetNRGBA(x, y, color.NRGBA{0, 0, 0, 0}) // 上半透明
+			} else {
+				src.SetNRGBA(x, y, color.NRGBA{12, 34, 56, 255})
+			}
+		}
+	}
+	var in bytes.Buffer
+	if err := png.Encode(&in, src); err != nil {
+		t.Fatalf("encode png: %v", err)
+	}
+
+	att, err := s.SaveImage(1, bytes.NewReader(in.Bytes()))
+	if err != nil {
+		t.Fatalf("SaveImage: %v", err)
+	}
+	if !strings.HasSuffix(att.URL, ".webp") || att.MIME != "image/webp" {
+		t.Fatalf("want webp attachment, got url=%s mime=%s", att.URL, att.MIME)
+	}
+
+	img, err := xwebp.Decode(bytes.NewReader(storedUploadPath(t, dir, att.URL)))
+	if err != nil {
+		t.Fatalf("decode stored webp: %v", err)
+	}
+	if _, _, _, a := img.At(0, 0).RGBA(); a != 0 {
+		t.Fatalf("alpha lost at transparent pixel: %d", a)
+	}
+	r, g, b, _ := img.At(5, 20).RGBA()
+	if r>>8 != 12 || g>>8 != 34 || b>>8 != 56 {
+		t.Fatalf("opaque color changed: %d,%d,%d", r>>8, g>>8, b>>8)
+	}
+}
+
+func TestSaveImageKeepsGIFAsIs(t *testing.T) {
+	s, dir := newUploadTestService(t)
+
+	src := image.NewRGBA(image.Rect(0, 0, 16, 16))
+	for y := 0; y < 16; y++ {
+		for x := 0; x < 16; x++ {
+			src.Set(x, y, color.RGBA{uint8(x * 16), uint8(y * 16), 0, 255})
+		}
+	}
+	var in bytes.Buffer
+	if err := gif.Encode(&in, src, nil); err != nil {
+		t.Fatalf("encode gif: %v", err)
+	}
+
+	att, err := s.SaveImage(1, bytes.NewReader(in.Bytes()))
+	if err != nil {
+		t.Fatalf("SaveImage: %v", err)
+	}
+	if !strings.HasSuffix(att.URL, ".gif") || att.MIME != "image/gif" {
+		t.Fatalf("gif should be kept as-is, got url=%s mime=%s", att.URL, att.MIME)
+	}
+	data := storedUploadPath(t, dir, att.URL)
+	magic := string(data[:6])
+	if magic != "GIF89a" && magic != "GIF87a" {
+		t.Fatalf("stored gif magic invalid: %q", magic)
+	}
+}
+
+func TestSaveImageKeepsWebPAsIs(t *testing.T) {
+	s, dir := newUploadTestService(t)
+
+	src := image.NewRGBA(image.Rect(0, 0, 20, 10))
+	var in bytes.Buffer
+	if err := webpenc.Encode(&in, src, webpenc.Options{Quality: ImageWebPQuality}); err != nil {
+		t.Fatalf("encode webp: %v", err)
+	}
+
+	att, err := s.SaveImage(1, bytes.NewReader(in.Bytes()))
+	if err != nil {
+		t.Fatalf("SaveImage: %v", err)
+	}
+	if !strings.HasSuffix(att.URL, ".webp") || att.MIME != "image/webp" {
+		t.Fatalf("want webp attachment, got url=%s mime=%s", att.URL, att.MIME)
+	}
+	if !bytes.Equal(storedUploadPath(t, dir, att.URL), in.Bytes()) {
+		t.Fatal("webp input should be stored byte-for-byte")
+	}
+	if att.Size != in.Len() {
+		t.Fatalf("size mismatch: att=%d input=%d", att.Size, in.Len())
+	}
+}