feat(plugin): add document export, doc‑block, and TTS/ASR support

Adds a DocBlock component that renders embedded documents, new export buttons for DOCX
and PDF, and updates the file‑upload picker to accept *.txt, *.docx, *.pptx, and *.pdf.
Introduces a DOCX→PDF conversion bridge in the backend and new /tts and /asr
endpoints that expose TTS and speech‑recognition functionality.  The README is
rewritten to describe the new features and clean up legacy documentation.  All
changes are backward‑compatible and do not introduce breaking API changes.

backend/docx2pdf_bridge.cjs

@@ -0,0 +1,20 @@
+const path = require('path')
+const { convert } = require('docx2pdf-converter')
+
+function main() {
+  const inputPath = process.argv[2]
+  const outputPath = process.argv[3]
+
+  if (!inputPath || !outputPath) {
+    throw new Error('缺少 DOCX 或 PDF 路径')
+  }
+
+  convert(path.resolve(inputPath), path.resolve(outputPath))
+}
+
+try {
+  main()
+} catch (error) {
+  console.error(error instanceof Error ? error.message : String(error))
+  process.exit(1)
+}