Add multi-file selection and bulk download to printer file browser

- Add checkbox selection for individual files
- Add Select All / Deselect All buttons
- Download single file directly, multiple files as ZIP archive
- Add POST /printers/{id}/files/download-zip endpoint
- Clear selection after successful download
- Support bulk delete for multiple files
- Update docs (changelog, wiki, website)

Closes #144

static/index.html

@@ -23,7 +23,7 @@
 
     <!-- Splash screens for iOS -->
     <link rel="apple-touch-startup-image" href="/img/android-chrome-512x512.png" />
-    <script type="module" crossorigin src="/assets/index-C0ILfTrK.js"></script>
+    <script type="module" crossorigin src="/assets/index-BwLAOG3i.js"></script>
     <link rel="stylesheet" crossorigin href="/assets/index-DVKqcow3.css">
   </head>
   <body>