Add updateassets command with webapp code detection

- Add updateassets command to update plugin files from embedded assets
- Only include webapp assets if plugin manifest indicates webapp code presence
- Compare file contents before updating to avoid unnecessary writes
- Display count of updated files in completion message

🤖 Generated with [Claude Code](https://claude.ai/code)

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

assets/webapp/.npmrc

@@ -0,0 +1 @@
+save-exact=true

assets/webapp/babel.config.js

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+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+const config = {
+    presets: [
+        ['@babel/preset-env', {
+            targets: {
+                chrome: 66,
+                firefox: 60,
+                edge: 42,
+                safari: 12,
+            },
+            modules: false,
+            corejs: 3,
+            debug: false,
+            useBuiltIns: 'usage',
+            shippedProposals: true,
+        }],
+        ['@babel/preset-react', {
+            useBuiltIns: true,
+        }],
+        ['@babel/typescript', {
+            allExtensions: true,
+            isTSX: true,
+        }],
+        ['@emotion/babel-preset-css-prop'],
+    ],
+    plugins: [
+        '@babel/plugin-proposal-class-properties',
+        '@babel/plugin-syntax-dynamic-import',
+        '@babel/proposal-object-rest-spread',
+        '@babel/plugin-proposal-optional-chaining',
+        'babel-plugin-typescript-to-proptypes',
+    ],
+};
+
+// Jest needs module transformation
+config.env = {
+    test: {
+        presets: config.presets,
+        plugins: config.plugins,
+    },
+};
+config.env.test.presets[0][1].modules = 'auto';
+
+module.exports = config;

assets/webapp/webpack.config.js

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+const exec = require('child_process').exec;
+
+const path = require('path');
+
+const webpack = require('webpack');
+
+const PLUGIN_ID = require('../plugin.json').id;
+
+const NPM_TARGET = process.env.npm_lifecycle_event; //eslint-disable-line no-process-env
+const isDev = NPM_TARGET === 'debug' || NPM_TARGET === 'debug:watch';
+
+const plugins = [
+    new webpack.ProvidePlugin({
+        process: 'process/browser',
+    }),
+];
+if (NPM_TARGET === 'build:watch' || NPM_TARGET === 'debug:watch') {
+    plugins.push({
+        apply: (compiler) => {
+            compiler.hooks.watchRun.tap('WatchStartPlugin', () => {
+                // eslint-disable-next-line no-console
+                console.log('Change detected. Rebuilding webapp.');
+            });
+            compiler.hooks.afterEmit.tap('AfterEmitPlugin', () => {
+                exec('cd .. && make deploy-from-watch', (err, stdout, stderr) => {
+                    if (stdout) {
+                        process.stdout.write(stdout);
+                    }
+                    if (stderr) {
+                        process.stderr.write(stderr);
+                    }
+                });
+            });
+        },
+    });
+}
+
+const config = {
+    entry: [
+        './src/index.tsx',
+    ],
+    resolve: {
+        alias: {
+            '@': path.resolve(__dirname, 'src'),
+        },
+        modules: [
+            'src',
+            'node_modules',
+            path.resolve(__dirname),
+        ],
+        extensions: ['*', '.js', '.jsx', '.ts', '.tsx'],
+    },
+    module: {
+        rules: [
+            {
+                test: /\.(js|jsx|ts|tsx)$/,
+
+                //exclude: /node_modules\/(?!(mattermost-webapp|@mattermost)\/).*/,
+                exclude: /node_modules/,
+                use: {
+                    loader: 'babel-loader',
+                    options: {
+                        cacheDirectory: true,
+
+                        // Babel configuration is in babel.config.js because jest requires it to be there.
+                    },
+                },
+            },
+            {
+                test: /\.(scss|css)$/,
+                use: [
+                    'style-loader',
+                    {
+                        loader: 'css-loader',
+                    },
+                    {
+                        loader: 'sass-loader',
+                        options: {
+                            sassOptions: {
+                                includePaths: ['node_modules/compass-mixins/lib', 'sass'],
+                            },
+                        },
+                    },
+                ],
+            },
+            {
+                test: /\.svg$/,
+                use: ['@svgr/webpack'],
+            },
+        ],
+    },
+    externals: {
+        react: 'React',
+        'react-dom': 'ReactDOM',
+        redux: 'Redux',
+        'react-redux': 'ReactRedux',
+        'prop-types': 'PropTypes',
+        'react-bootstrap': 'ReactBootstrap',
+        'react-router-dom': 'ReactRouterDom',
+    },
+    output: {
+        devtoolNamespace: PLUGIN_ID,
+        path: path.join(__dirname, '/dist'),
+        publicPath: '/',
+        filename: 'main.js',
+    },
+    mode: (isDev) ? 'eval-source-map' : 'production',
+    plugins,
+};
+
+if (isDev) {
+    Object.assign(config, {devtool: 'eval-source-map'});
+}
+
+module.exports = config;