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),