feat(preview): add file preview page with metadata and styling

Implement a rich file preview interface to allow users to view file
contents directly in the browser.

Changes include:
- Exposing raw file size (`SizeBytes`) in the download handler's file view.
- Adding comprehensive CSS styling for the preview layout and cards.
- Integrating Prism.js for syntax highlighting of code files.
- Updating Content Security Policy (CSP) headers to permit inline styles and frame sources required by the preview components.
- Adding unit tests to ensure preview metadata attributes are correctly rendered on the download page.

backend/libs/handlers/download.go

@@ -40,6 +40,7 @@ type fileView struct {
 	ID           string
 	Name         string
 	Size         string
+	SizeBytes    int64
 	ContentType  string
 	PreviewKind  string
 	URL          string
@@ -404,6 +405,7 @@ func (a *App) fileViewWithReactions(box services.Box, file services.File, reacti
 		ID:           file.ID,
 		Name:         file.Name,
 		Size:         helpers.FormatBytes(file.Size),
+		SizeBytes:    file.Size,
 		ContentType:  file.ContentType,
 		PreviewKind:  file.PreviewKind,
 		URL:          fmt.Sprintf("/d/%s/f/%s", box.ID, file.ID),

backend/libs/handlers/upload_stage3_test.go

@@ -151,6 +151,34 @@ func TestSocialPreviewBotGetsRawFilePreview(t *testing.T) {
 	}
 }
 
+func TestFilePreviewPageIncludesPreviewMetadata(t *testing.T) {
+	app, cleanup := newTestApp(t)
+	defer cleanup()
+	payload := uploadThroughApp(t, app)
+
+	request := httptest.NewRequest(http.MethodGet, "/d/"+payload.BoxID+"/f/"+payload.Files[0].ID, nil)
+	request.SetPathValue("boxID", payload.BoxID)
+	request.SetPathValue("fileID", payload.Files[0].ID)
+	response := httptest.NewRecorder()
+	app.DownloadFile(response, request)
+
+	if response.Code != http.StatusOK {
+		t.Fatalf("status = %d, body = %s", response.Code, response.Body.String())
+	}
+	body := response.Body.String()
+	for _, want := range []string{
+		`data-size-bytes="5"`,
+		`data-source-url="/d/` + payload.BoxID,
+		`data-download-url="/d/` + payload.BoxID,
+		`data-icon-url="/static/file-icons/`,
+		`data-preview-tabs`,
+	} {
+		if !strings.Contains(body, want) {
+			t.Fatalf("preview page missing %q: %s", want, body)
+		}
+	}
+}
+
 func TestResumableUploadFlowCreatesNormalBox(t *testing.T) {
 	app, cleanup := newTestApp(t)
 	defer cleanup()