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/middleware/security.go

@@ -9,7 +9,7 @@ func SecurityHeaders(next http.Handler) http.Handler {
 		header.Set("X-Frame-Options", "DENY")
 		header.Set("Referrer-Policy", "strict-origin-when-cross-origin")
 		header.Set("Permissions-Policy", "camera=(), microphone=(), geolocation=()")
-		header.Set("Content-Security-Policy", "default-src 'self'; img-src 'self' data: blob:; media-src 'self' blob:; font-src 'self'; style-src 'self'; script-src 'self'; base-uri 'self'; frame-ancestors 'none'")
+		header.Set("Content-Security-Policy", "default-src 'self'; img-src 'self' data: blob:; media-src 'self' blob:; font-src 'self'; style-src 'self' 'unsafe-inline'; script-src 'self'; frame-src 'self' about:; base-uri 'self'; frame-ancestors 'none'")
 
 		next.ServeHTTP(w, r)
 	})