Move plugins to manifest, pin Docker version, add Makefile

- Add plugins.txt listing all plugins for reproducible installs
- Add Makefile with setup/start/stop/install-plugins targets
- Remove user/plugins/ from git tracking
- Pin Docker image to 1.7.49.5-ls244

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

plugins/admin/themes/grav/app/utils/formatbytes.js

@@ -1,11 +0,0 @@
-const sizes = ['Bytes', 'KB', 'MB', 'GB', 'TB', 'PB', 'EB', 'ZB', 'YB'];
-
-export default function formatBytes(bytes, decimals) {
-    if (bytes === 0) return '0 Byte';
-
-    let k = 1000;
-    let value = Math.floor(Math.log(bytes) / Math.log(k));
-    let decimal = decimals + 1 || 3;
-
-    return (bytes / Math.pow(k, value)).toPrecision(decimal) + ' ' + sizes[value];
-}