fix: remove dead delete+recreate and pin code, add poll fallback test

Phase 1 cleanup:
- Remove deletePost() method (dead code, replaced by PUT in-place updates)
- Remove _postInfo Map tracking (no longer needed)
- Remove pin/unpin API calls from watcher-manager.js (incompatible with PUT updates)
- Add JSDoc note on (edited) label limitation in _flushUpdate()
- Add integration test: test/integration/poll-fallback.test.js
- Fix addSession() lastOffset===0 falsy bug (0 was treated as 'no offset')
- Fix pre-existing test failures: add lastOffset:0 where tests expect backlog reads
- Fix pre-existing session-monitor test: create stub transcript files
- Fix pre-existing status-formatter test: update indent check for blockquote format
- Format plugin/ files with Prettier (pre-existing formatting drift)

plugin/webapp/webpack.config.js

@@ -0,0 +1,32 @@
+var path = require('path');
+
+module.exports = {
+  entry: './src/index.tsx',
+  output: {
+    path: path.resolve(__dirname, 'dist'),
+    filename: 'main.js',
+    library: {
+      type: 'umd',
+    },
+  },
+  resolve: {
+    extensions: ['.ts', '.tsx', '.js', '.jsx'],
+  },
+  module: {
+    rules: [
+      {
+        test: /\.tsx?$/,
+        use: 'ts-loader',
+        exclude: /node_modules/,
+      },
+      {
+        test: /\.css$/,
+        use: ['style-loader', 'css-loader'],
+      },
+    ],
+  },
+  externals: {
+    react: 'React',
+    'react-dom': 'ReactDOM',
+  },
+};