feat(import): one Import-file entry + vault notebooks

Phase 3. Import becomes a single top-level action beside "Create
notebook" and the library is vault-backed.

- VaultService.createNotebook copies a picked file into a fresh
  (de-duplicated) notebook folder under the vault; its sidecar lives
  beside it, so annotations travel with the file.
- Home screen: one "Import file" action with a multi-extension picker
  (pdf / docx / pptx); routes to the editor by extension.
- The document list is now a vault scan (folders with a source file),
  not the SQLite documents table — no cache, always correct.
- PPTX soffice detection fix; DOCX convert-on-import is best-effort
  and fails gracefully when LibreOffice is unavailable.

analyze clean, tests green.

lib/l10n/app_zh.arb

@@ -5,6 +5,13 @@
   "search": "搜索",
   "importPdf": "导入 PDF",
   "importPpt": "导入 PPT",
+  "importFile": "导入文件",
+  "createNotebook": "新建笔记本",
+  "noDocumentsYet": "暂无文档——点按“导入文件”",
+  "processingImport": "正在导入…",
+  "importFailed": "无法导入该文件：{error}",
+  "convertNeedsLibreOffice": "导入 Word 文档需要安装 LibreOffice。请先转换为 PDF，或安装 LibreOffice。",
+  "unsupportedFileType": "不支持的文件类型：{ext}",
   "penCanvasBeta": "手写画布（测试版）",
   "newNote": "新建笔记",
   "open": "打开",