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_en.arb

@@ -5,6 +5,19 @@
   "search": "Search",
   "importPdf": "Import PDF",
   "importPpt": "Import PPT",
+  "importFile": "Import file",
+  "createNotebook": "Create notebook",
+  "noDocumentsYet": "No documents yet — tap Import file",
+  "processingImport": "Importing…",
+  "importFailed": "Couldn't import that file: {error}",
+  "@importFailed": {
+    "placeholders": { "error": { "type": "String" } }
+  },
+  "convertNeedsLibreOffice": "Importing Word documents needs LibreOffice installed. Convert to PDF first, or install LibreOffice.",
+  "unsupportedFileType": "Unsupported file type: {ext}",
+  "@unsupportedFileType": {
+    "placeholders": { "ext": { "type": "String" } }
+  },
   "penCanvasBeta": "Pen Canvas (beta)",
   "newNote": "New Note",
   "open": "Open",

lib/l10n/app_localizations.dart

@@ -128,6 +128,48 @@ abstract class AppLocalizations {
   /// **'Import PPT'**
   String get importPpt;
 
+  /// No description provided for @importFile.
+  ///
+  /// In en, this message translates to:
+  /// **'Import file'**
+  String get importFile;
+
+  /// No description provided for @createNotebook.
+  ///
+  /// In en, this message translates to:
+  /// **'Create notebook'**
+  String get createNotebook;
+
+  /// No description provided for @noDocumentsYet.
+  ///
+  /// In en, this message translates to:
+  /// **'No documents yet — tap Import file'**
+  String get noDocumentsYet;
+
+  /// No description provided for @processingImport.
+  ///
+  /// In en, this message translates to:
+  /// **'Importing…'**
+  String get processingImport;
+
+  /// No description provided for @importFailed.
+  ///
+  /// In en, this message translates to:
+  /// **'Couldn\'t import that file: {error}'**
+  String importFailed(String error);
+
+  /// No description provided for @convertNeedsLibreOffice.
+  ///
+  /// In en, this message translates to:
+  /// **'Importing Word documents needs LibreOffice installed. Convert to PDF first, or install LibreOffice.'**
+  String get convertNeedsLibreOffice;
+
+  /// No description provided for @unsupportedFileType.
+  ///
+  /// In en, this message translates to:
+  /// **'Unsupported file type: {ext}'**
+  String unsupportedFileType(String ext);
+
   /// No description provided for @penCanvasBeta.
   ///
   /// In en, this message translates to:

lib/l10n/app_localizations_en.dart

@@ -23,6 +23,32 @@ class AppLocalizationsEn extends AppLocalizations {
   @override
   String get importPpt => 'Import PPT';
 
+  @override
+  String get importFile => 'Import file';
+
+  @override
+  String get createNotebook => 'Create notebook';
+
+  @override
+  String get noDocumentsYet => 'No documents yet — tap Import file';
+
+  @override
+  String get processingImport => 'Importing…';
+
+  @override
+  String importFailed(String error) {
+    return 'Couldn\'t import that file: $error';
+  }
+
+  @override
+  String get convertNeedsLibreOffice =>
+      'Importing Word documents needs LibreOffice installed. Convert to PDF first, or install LibreOffice.';
+
+  @override
+  String unsupportedFileType(String ext) {
+    return 'Unsupported file type: $ext';
+  }
+
   @override
   String get penCanvasBeta => 'Pen Canvas (beta)';
 

lib/l10n/app_localizations_zh.dart

@@ -23,6 +23,32 @@ class AppLocalizationsZh extends AppLocalizations {
   @override
   String get importPpt => '导入 PPT';
 
+  @override
+  String get importFile => '导入文件';
+
+  @override
+  String get createNotebook => '新建笔记本';
+
+  @override
+  String get noDocumentsYet => '暂无文档——点按“导入文件”';
+
+  @override
+  String get processingImport => '正在导入…';
+
+  @override
+  String importFailed(String error) {
+    return '无法导入该文件：$error';
+  }
+
+  @override
+  String get convertNeedsLibreOffice =>
+      '导入 Word 文档需要安装 LibreOffice。请先转换为 PDF，或安装 LibreOffice。';
+
+  @override
+  String unsupportedFileType(String ext) {
+    return '不支持的文件类型：$ext';
+  }
+
   @override
   String get penCanvasBeta => '手写画布（测试版）';
 

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",